Right now, Microsoft released its free security program, Microsoft Security Essential. It can not work on Windows 2000.
Operating System Requirement: Genuine Windows XP (Service Pack 2 or Service Pack 3); Windows Vista (Gold, Service Pack 1, or Service Pack 2); Windows 7
Let's see what he said about it:
Microsoft Security Essentials provides real-time protection for your home PC that guards against viruses, spyware, and other malicious software.
I am not very sure how good it is. You would better try it yourself and leave your comment to tell others how it works.
You can get it free here.
When i read it,firstly i use it and really it works very nice..its real protection of my PC.
I just installed it, and let me tell you, I'm quite surprised with it, it works pretty well.