Sigh, i wish people would use the search function.
Microsoft Security Essentials
Out of those, Avast! is the best - it's real-time shield makes it second to none at preventing you getting viruses. Everytime i've ever accidentally clicked a virus link, it immediately blocks it, tells me what it is and where it came from. MSE is the most common, but it's real-time protection is crap IMO - it's good at finding viruses, not so good as keeping them out. Obviously some of those are paid.
Best used in combination with: