Get this fella, it's free and good:
http://www.microsoft.com/security_essentials/
Also, stop using Internet Explorer, get one of these:
http://www.mozilla-europe.org/en/firefox/
http://www.google.com/chrome
http://www.opera.com/download/
TBH there's not a lot to choose between them now, the last one, Opera, is probably safest as it's the least used and so less targeted.