For protecting against and removing Viruses, Torjans, Worms etc...
Avast! Antivirus v4.7 Home is entirely free. You can get it here:
Alternatively, AVG Antivirus is also free:
I prefer Avast! than AVG. No specific reason...I just prefer the look.
For removing Adware and Spyware...
Spybot Search & Destroy is a free, very popular program:
Alternatively, Lavasoft Adaware SE Personal: