There are a number of 'fake av' products around at the moment that infect your computer tell you that you have lots of viruses on your computer and try to get you to sign up to their service which does nothing. (see <a class="postlink" href="http://krebsonsecurity.com/2010/07/rogue-antivirus-victims-seldom-fight-back/" onclick="window.open(this.href);return false;">http://krebsonsecurity.com/2010/07/rogu ... ight-back/</a>)
Try microsoft security essentials as a free av system it seems to be one of the best on various comparisons.
<a class="postlink" href="http://www.microsoft.com/security_essentials/?mkt=en-us" onclick="window.open(this.href);return false;">http://www.microsoft.com/security_essentials/?mkt=en-us</a>
Most malware uses well known problems which have had patches issued to fix them. It is important to ensure you are running microsoft updates and also updating software such as adobe pdf reader and flash when prompted. Adobe plug ins are one of the main targets for cyber criminals.