This is mainly so i can link it to my buddies, but uh, here is a lot of the programs i use to protect/maintain my pc.
CCleaner: Cleans registry, and cleans crap off of your temp files.
Defraggler: Best program to fix fragments on your HDD, keeps it fast and quick.
AVG 8.0 Free: Simple, easy Anti virus. Simple as that.
COMODO Firewall Pro: Best free software firewall IMHO. It basically asks you about everything that your pc will do at any given time.
Best Anti Spyware: Spybot S&D. It cleans up alot of crap on your pc. simple, easy.
ALL of those programs are free. Just keep them updated regularly, i enjoy them.