Avast 5 is the latest version of a great (and free) antivirus program. It’s designed for Windows PCs and does a pretty good job. And did I mention it’s free? There’s no excuse not to have an antivirus program installed on any computer.
There’s also a paid version which extra features such as a personal firewall. I personally use and trust Trend Internet Security, but Avast would be my other choice (especially the paid version called Avast Internet Security).
The free version can be downloaded from here.