How much memory do you have installed? If you are using most of it windows will have to continuously swap data from your memory out to disk and back to run multiple processes. Unfortunately disk access is much slower than memory access, so if you have a few programs/processes running which need more memory than your system can provide, say P1 and P2 it will run P1 for a little bit (milliseconds), than swap P1's data out to disk and swap P2 back in and so forth. If this happens your computer will slow way down since it will be continuously swapping to disk.
Anyway if your level of memory use is very high and you can hear your disk working all the time, this should be your problem. Best solution would be to get a memory upgrade, try mcglen.com they have very reasonable prices. Check streetprices.com also if you want to shop around for the best prices.
Hope this helps
