I build/repair computers professionally, I just wanted to recommend two programs that go on EVERY PC I work on:
Spyware Blaster - Super program and takes virtually NO resources. All it does is add bad websites to your restricted websites list. No background process running and it blocks malware/viruses at the source. Only downside, the free version requires manual updates, but it's just a couple clicks of the mouse:
http://www.javacoolsoftware.com/spywareblaster.html
CCleaner - Amazing program that will clean all the crap out of your system with just a click. One of the biggest problems I see on PC's I fix is temp folders filled with garbage. The vast majority of PC users have no idea that these folders should be periodically cleaned out to keep your system running fast. Just watch the install, it will opt to install Google desktop or toolbar and automatically check for updates (not really necessary) by default, I recommend unchecking those boxes during setup.
http://www.ccleaner.com/
Enjoy and thank me when your system is running faster!