1) The first thing to is choose which platform (processor) you want, AMD or Intel based on your needs. Read some reviews on the different Processors & choose one based on your needs. But the Best by far at the momment are the Intel Core 2 Duo range & the lower end ones can be had for under £100
2) Choose your Graphics Card, The two choices are ATI or Nvidia, Do you want a single or duel Card setup (only needed for extreme gamers). The choices are SLI for Nvidia or Crossfier for ATI, more details about SLI & Crossfire are below in the Motherboard section. Choose a card based on your needs, No point in spending £300 for a card to do word processing. I prefer Nvidia ( I have tried both types). The newer Cards run in PCIx16 slots old ones in AGP x8 or x4. Pcix16 is by far the best option. The latest cards support DirectX 10 but Vista is needed, It is worth getting a DX10 card though.
3) Choose your Motherboard, two of the most important things to consider are the CPU (Processor) Socket & Chipset, The best ways to do this are 1) Choose the Processor & determine which Socket it requires & the FSB speed, the newer Core 2s run at 1333 FSB so be sure to get a Board that supports it. 2) Are you are going for an ATI or Nvidia Card & weather you would like to add a second Graphics Card now or in the future, These technologies are called Crossfire for ATI & SLI for Nvidia, In my opinion SLI is better as both cards run at full speed, ATI Crossfire fun at x16 for card 1 but only x4 for the second card apart from the new Asus Intel X38 & P35 chipset boards with Crosslinx which run both Cards at x8 speeds. There are a wide variety of Chipsets on the market, The best thing to do is read about them based on your ATI or Nvidia choice, The Nvidia SLI technology only runs on Nvidia Chipset boards the latest being the 680i Chipset. Another factor to consider is the RAM the latest Intel Chipsets support DDR3 but DDR3 is very expensive & has high latencys.
4) Choose your RAM (Memmory) to do this find out what RAM your motherboard supports & read some reviews. The best makes are Corsair, Mushkin, Crucial, OCZ & Kingston. 2Gig is the most popular amount & I think is sufficient. Also be sure to get the right RAM most boards take DDR2.
5) Choose the Hard Drive. Make sure you get the right interface for your Motherboard IDE (older) or SATA (new). SATA is better & some new boards dont support IDE. I rate Western Digital as the best make. The Hard Drive is the "Bottleneck" in a PC so get the best you can afford. Go for performance over storage capacity. The more cache the better, the better models have 16mb. I have a Western Digital Raptor X, it is the fastest & best Hard Drive you can buy for a desktop PC. They are pricey but worth the money. Also consider if you want one or more Hard Drives, The Raptors have a maximum of 150GB so I have 2 running in Raid 0. Most new Motherboards have a built in Raid Controller but be sure to check if you want Raid. The advantage of Raid is speed, 2 drives in Raid perform much better that a single drive or two drives not in Raid. Read up on Raid if you going to consider it as there are a few options & each has its own pros & cons.
4) Choose your PSU (power Supply). Dont skimp on the Power Supply get the best you can afford. The system you build depends on how much power you need but the new graphics card require a lot of power. Get a minimum of 600W but more is better, I have 750W & think that is sufficient. Again read some reviews. The rated Wattage of a PSU is not always honest, They sometimes rate it at the Highest Peak, so a 600W is really a 500W but can reach 600W. It really is better to go for a good, well known quality manafacturer. I have a Thermaltake Toughpower, they are reasonably priced & are very good. Some PSUs come with modular cabeling which allows you to use only the cabels you need & is good for Case tidiness & Airflow.
5) Cases these days are not like the old boring plain white plastic cases of old, there are lots of very cool looking cases, watercooled cases & some with huge 25cm fans like the Aplus twin engine case to choose from. The challenge is to balance looks, practicality, quality, cooling ability & noise levels, although you can change the fans for quieter better quality ones quite easily. Choose a case, based on your motherboard size, a desktop uses ATX for this you will need a tower. Things to look for are 1) Airflow & Fan accomodation, good airflow is important. 2) Case size (if you intend to add watercooling you may want a case with more room). 3) Tool-less design, some cases have tool-less expansion card, bay installation & side pannel removal. 4) Side window, for asthetic purposes you may want a window to show of your high-end Hardware. 5) Number of bays internal & external, all cases have a different number of 5.25 Drive bays (for dvd drives) & 3.25 drive bays (for floppy disc). The number of external 5.25 bays may be important if you intend to cool your hard drives or add a water cooling 5.25 resevoir add more than one DVDrw etc. 6) front ports like headphones, USB, firewire etc maybe important to you. 7) Case quality. there are alot to choose from steel, aluminium, perspex see through & all vary in quality. It is best not to skimp on this part as you might regret it later if you want to "Mod" the case with a top Blowhole or side Fan etc. Again read reviews & search google images for PC Cases or Gaming Cases, you will be amazed at the choice. A good case can be had for around £80 although if it comes with a PSU I always remove it & add my own so try get the Case without the PSU. Watercooled cases are around £150 & are good, Thermaltake make good ones, They used to have a bad rep for their watercooling kits but that has changed drasticly. They are now very good & reasonably priced.
6) Cooling. This is a very important part of the Build which is often overlooked. Cooling the Hardware such as the CPU, Graphics Card, RAM & hard Drive not only allow for better overclocks but will increase the life span of the components. The options are 1) Air Cooling can be OK with a good quality cooler 2) Watercooling, far superior to Air Cooling it can be quieter & passive (fanless) systems are available. A Kit can be had for £50 upwards. Again it is better to go for a quality kit, There are a few manafacturers to choose from, Koolance, DangerDen, Swiftech & Thermaltake are good options, or even better build your own, You will need a CPU Block (Graphics, Chipset, RAM & Hard Drive Blocks can also be used) Pump, Resevoir, Raidiator, Tubbing, Coolant & fittings. Search the Web, Forums & read reviews to buy according to your requirements. Dont be put off by leaks as Non Conductive Coolant is available (PCIce is a good one) & if fitted properly will be fine & give you great Cooling performance & Overclocking potential. 3) TEC (Thermoelectric Cooling) also knows as Peltier, These give excellent cooling but have a downside, You will need watercooling to cool the TEC, they act as heat pumps pumping heat from one side to the other & need something to remove the heat from the hot side, As they cool below ambient condensation can be caused & precautions need to be taken for this, Coat the Motherboard PCB (printed Circuit Board) with conformal coating seal the socket with neopreme & dielectric grease, Instructions can be found on the internel easily enough & done properly should prevent any condensation from forming & will protect against damage if it does. Swiftech make TEC CPU & graphics card Blocks, If done corectly this is a very effective cooling method & can produce sub zero temperatures. A seperate Power Supply for the TECs might be required as they are amp hungry. Again information & answers to any questions you need can be found on various forums. It is not a cheap method of Cooling as is usually only indulged by enthusiasts but has very good advantages 4) Phase Change Cooling, Again this has the same condensation issues as TEC Cooling but can be overcome by the same methods above. It is an expensive method of Cooling but will produce sub zero temperatures which allow for extreme overclocking & again only indulged by enthusiasts, Search for Vapochill for more information.
7) Put all the components together starting with placing the Motherboard into the Caes, but be sure to check weather your CPU cooler needs a back plate installed, Then install all the other Hardware according to the instructions. Install all the cabels & the coolers then switch it on & install the Operating System. Sit back & admire all your handy work, brag to your friends & colleagues & wollow in the satisfaction you will get from it.
Thats it, It is pretty simple really as long as you make sure all the Hardware is compatable with each other. Every component usually comes with installation instructions, it is one big glorified puzzle. None of the components can be inserted into the wrong place as they will not fit anywhere else than where they are meant to be.
I hope this guide has been of some use to you. There is allot of satisfaction to building your own system, it will be better & cheaper than anything in the Shops & you will know the system inside out. All of the Hardware will come with its own guarantee so dont worry about that. I would never ever buy an off the shlelf PC again. A good Pc can be built for around £250-£300 maybee cheaper with used hardware from eBay.