Two reasons why computers slow down is . . .
1. Because of a lack of RAM - you have too much going on even when you aren't doing much. As well as needing a decent processor speed (of at least 1.5 Ghtz), you also need at least 512MB of RAM. You can usually check how much RAM a computer has if you go on 'My Computer' and then, to the left, hit 'View System Information' (if using Windows XP).
Right click on anywhere near the bottom right hand clock and click 'Task Manager'. On 'Processes' it will show you what is using up your RAM and then you can click what you want to remove. You will need some stuff so the whole thing will be a case of trial and error - if you need something, the actions are reversable and will come back next time you log in.
2. You may have registry problems - I suggest downloading something free and quick like 'Crapcleaner'. This will remove the rubbish in your computer and fix potential problems that causes crashing.
http://www.ccleaner.com/ccdownload.asp
Good luck.
