Don't use IE. Don't use Outlook. Don't download anything dodgy.
That's worked for me for the last 2 years. No viruses or Malware on my Windows 7 system and it's still running quick.
I do have an image I made after I installed all the drivers and core software I use, so easy enough to restore it to a good state if something does go wrong.
I find all these programs that claim to clean your registry, prevent malware and such are just a headache. They are always popping up and wanting to do something or install an update, and they probably contribute to the system slowing down.
Ideally I'd run Mac OS X as my main OS and keep Windows virtualised, but I'm reluctant to pay the premium for apple hardware now and setting up a Hackintosh is just a ball ache I can't be bothered with these days.
