This little tip is gem considering the number of nasties getting around the internet at the moment. As many people know, virus scanners on your hard drive are only useful if you keep your definitions up to date. For Norton Antivirus, this happens every week, usually on a Wednesday, through it's LiveUpdate service, but some viruses are spreading in the wild within a couple of days, able to infect you in the time gap.
If you don't have a scanner, or want to be doubly sure, you can use a free online virus scanner at http://housecall.antivirus.com. This tool, managed by anti-virus company Trend Micro is accessed through your web browser, and after a few minutes, it can be scanning your whole computer for viruses and other threats, for free.
If you think you've been infected, go to this site, follow the instructions, and scan your machine for viruses and tips on how to repair them.