Man, oh man, you guys all need to do yourselves a favor and buy a Mac, especially now you can get a Mac Mini for $399 and use your existing monitor, mouse and keyboard. I've never heard of a single Mac virus, and where MS Windows XP comes with all ports open, and lots of back doors, MacOS X is bulletproof. I would like to think Jon could back me up on this although a lot of people have differing opinions on this matter.
I use both, and much prefer the Mac hardware and OS, but I can also testify to how good adaware is (www.adaware.com) if you are running a Windows platform. I have had a Dell Inspiron 8600 notebook that has been running a single install of Windows XP Pro for over a year now, it never slows down, and I never get annoying popups.
Heres some tips:
1. Keep the registry clean (use a program like regcleaner which is freeware, just do a google search for it).
2. Install a spyware remover like adaware and scan your system regularly (I have to remove up to 15 malicious spyware programs each day that automatically install themselves just by being online).
3. Install some decent antivirus software.
4. Make sure you maintain a good amount of working free space on your hard drives (10% is a good idea, so this would mean keeping 10GB free on a 100GB drive), Windows (or any OS for that matter) needs this breathing space.
5. Defragment your hard drives regularly (for WinXP go to your start menu, click all programs, go to accessories, then system tools, then click disk defragmenter).
If you do these things, you will have a happy, fast computer that should keep going indefinitely without problems, or buy a Mac, don't worry about any of the above and then laugh at your friends when thier PC's grind to a halt every other week.
Hope this helps!