Windows Defender is a free program that helps you stay productive by protecting your computer against pop-ups, slow performance and security threats caused by spyware and other potentially unwanted software. Windows Defender no longer supports Windows 2000 as mainstream support ended in June 2005.
My wife uses it on her laptop, since the beta days, and it actually performs quite well!
I haven't tried it yet as I still use Symantec as well as other options, but she swears by it (but never AT it).