Viruses are always a threat to your computer. New ones are being created almost daily by people who are sick or are terrorists or who are drawn by the challenge of creating a program that can reproduce itself and spread itself and do this without being detected.
Some of them do not intend to cause damage but many do. In any case, you need to be protected.
All of the anti-virus program sites try to keep on top of current threats. Symantec, McAfee and Trend Micro (among many others) will give you information about the latest virus threats and what to do if you are (or suspect you are) infected.
The threat of viruses is very real. Every computer user should use some kind of virus-protection software and keep it updated regularly - as often as the software's author provides updates. This used to be monthly but with the proliferation of new viruses, many are doing it at least weekly. This is an indication of how active the virus authors are - protect yourself accordingly