Nowadays, anti-virus protection is not an option, it is indispensable. That's why Microsoft now provides a new protection tool for Windows users. I'm talking about the new Microsoft Security Essentials, a powerful anti-virus & anti-spyware tool.
Before installing the program, as it's usual with other Microsoft products, the setup process will validate if your Windows copy is genuine. After that, the program will download the latest definitions of viruses & spyware.
Compared to other anti-virus programs, Microsoft Security Essentials works pretty well and fast. The update process is performed automatically (although it can be done manually too). Scheduled scanning tasks can be set as well. Besides, it doesn't use a lot of resources of your system, in fact, the resources used are minimal.
One feature that I personally consider necessary in a good anti-virus program is the boot-time scan, that is, a scanning process performed before Windows starts. Unfortunately, this program doesn't have it.
Apart from that, I can say that Microsoft Security Essentials is a competitive anti-virus & anti-spyware tool. You can download and use it for free.