Since most of you are using Windows, it's mandatory to use a good virus scanner (that is: a paid one) - the free ones don't offer the same protection level and frequency of pattern files updates. I advice not to use a suite like Norton Internet Security or similar - they use way too many resources and only lure you into believeing that there is such a thing as a full security. The most important component of the virus protection chain resides between your ears though.
The tool that Hein initially recommended is not a surrogate for a real virus scanner that permanently resides on your PC - it's an extra tool that can be usefull if your PC acts "funny" although you have a virus scanner and you therefor want a second opinion of another virus scanner. Such external extra scanners are available from some other virus scanner vendors as well (like TrendMicro's HouseCall). They are meant to be used to scan your PC for infections that might bypassed your local scanner, so they can be helpfull under certain conditions. I use such tools from time to time (I work as a sys admin for a living) - usually, if one of my roaming users who are away for longer from the company return home and complain about their notebooks acting up strangely.
The "real" anti virus products I can recommend (although each of them has got indvidual weaknesses and strengths) are:
The ones I can not recommed are:
There is a load of other scanners that I have no experience with.