Just a minor mention also to everyone... Most people recommend MSE because it is free, comes with Windows 7.
However, the rule of thumb with ANY AV/AM based software, even I harp this to people, is that it doesn't protect you 100% of the time. None of them do. There will ALWAYS be a new version of a trojan, virus or malware that all AV/AMs are not aware of until it is too late.
Blindly trusting ANY AV/AM program to keep you protected is like expecting the an alarm system to stop a home invader from breaking in and doing a quick grab. Sure, it will get the cops sooner to your location, assuming the Alarm company is on their toes or you made sure you had it set, but doesn't mean a prepared person won't hit you all the same.
The best AV/AM system available is you and safe browsing. Safe Browsing means not getting questionable material or going to websites promoting non-legit material as they are often going to be the easiest vectors to attack a computer, especially for people who have not learned to be a little paranoid about where they go.
Using restrictive web browsing, like Firefox with NoScript and Ad-blocker goes a long way in making it a bit more difficult when accidentally tripping over sites, but again, if you set things too loose, you might as well just leave your computer without a firewall or anti-virus as you are just inviting trouble by not being careful.