Windows Security Essentials.
Windows Firewall.
Firewall in my router (DD-WRT).
Spybot (for registry cleaning).
Not downloading dodgy things from the internet (be clever).

Those are the things that I use. It's worked out okay so far, especially the last one. Viruses don't just magically appear on your computer. Most of the time they're hidden in things you download, or downloaded onto your system from dodgy websites that you use. Rarely is it the case that someone directly sends you a virus.