If you have Windows XP and you are not connected to the internet and have no Anti Virus or Anti Spyware at all then 256Mb is enough.
If you have Windows XP and you have a full Internet Security suite, such as Panda, then you must have 512Mb.
If you use your PC for Video Editing then you should have 1Gb of RAM.
To tell how much your PC is using then look at the Task Manager in Windows XP or the System Monitor in earlier versions of Windows.
If your PC is actively using the Swap File / Paging Space then it does not have enough RAM.
If you have plenty of RAM and your PC is displaying messages about being low on system resources then it is almost certainly infect a Virus or some Spyware.
For Vista and Windows 7 you need 512b just to install the software, we recommend 2Gb of RAM.
If you have any questions then feel free to visit our eBay Shop : Highridge Computers Ltd.