Please give us details of some of your PC components, for example:
Processor + socket type, RAM and what type it is [i.e pc2700, pc3200 etc etc] also how much ram [i.e 2x 512 MB], video card to name a few.
Have you also tried speeding it up with doing some virus scans, spyware scans, disk defragging. If not, I highly reccomend doing one now, some good programs to try:
Anti - Virus: AntiVir, AVG FREE
Anti - Spyware: Spybot - Search and Destroy, Super AntiSpyware free Edition and Ad-Aware SE Personal.
[I advise you to download atleast two of each category, even if one program doesn't detect something, you have a chance the other will]
I also highly highly highly reccomend you downloading a program called CC cleaner and doing a run, if you haven't heard of it before, and it is your first time doing a scan with CC-Cleaner, then the results you'll get with it are outstanding [the program clears up old files and stuff you don't need, it once cleared up 3k MB of stuff on one of my older pcs]