If your PC is slowing down then it's got too much junk running on it.
On a newly formatted and loaded XP system there are about 40+ programs running in the back ground. Everytime you load a program that decides it has to be "ready" and parks itself in you system tray it means your systems resources are being used. Some devices seem innocous, like a printer, but the printer software can be bogging your computer down.
Now if the computer is hooked to the 'net, add in the spyware that is being loaded on your computer as you surf and it's going to come to a crawl.
So first things first....
http://www.lavasoft.com then download Adaware personal, its free. If your computer is connected by dsl or cable you want to get a firewall running too.
If your computer is 3 years old and freezing up its more than likely an overheating problem. Map out the cables, disconnect them. UNPLUG THE COMPUTER FROM THE POWER (please do not get electrocuted). Open the case and vacuum the computer out using a plastic or rubber tipped vacuum, if you have a shop vac that blows you can blow it out too. While you're at it vacuum the keyboard. Using your map put things back together.
This should give you a head start. PC or Mac is like a religious argument, I use a PC but it runs Linux instead of Windows. If you use a PC at work, I'd stay PC at home. I don't know who is worse Bill Gates or Steve Jobs.
Dave