The key thing to remember when buying a new (or refurbished) computer is what functions you need it to perform and don't get tempted into buying something with more processing power than it needs to have. If you're not technically minded and don't know the exact specification that you require then at least have an idea in your mind as to exactly what you want to do with the computer.
For example, someone who is only going to type the occassional email and word processed document does not need the same kind of processing power or equipment that someone running a video editing business requires. Don't get tempted into buying more than you need and don't be fooled by added extras such as webcams, software, etc which are not on your list.
A reputable company should always give you impartial advice, but bear in mind that the salesman may well be on commission and hence has a vested interest in pushing you towards the higher specification higher cost machines.
Using sites such as eBay to research your purchase is often an excellent way of identifying how much specific computers are worth rather than how much vendors would like to sell them for. In the current climate, there are amazing bargains to be found as companies seek to downsize and sell on excess stock, plus bankrupt stock from companies who are unable to survive in the current financial climate.
How about the deals where you get a free laptop in exchange for signing a two year contract with a mobile telecomms provider? These tend to be low specification laptops or mini notebooks which tend to be half the price of the two year contract ... and do you really need access to your email and the Internet all of the time? You also need to read the fine print to ensure that you have an adequate monthly allowance for your data requirements and how much it will cost you per megabyte or gigabyte over that original quota. And make sure that you monitor your usage adequately, otherise you could easily find that the afforable monthly payment you envisaged is twice or four times more than what you were expecting.