Lately some people have been victimized by a newer scam, one made possible by the remote access features in Windows 7. Here's how it works --
Someone calls you on the phone, claiming to be from Microsoft or from Norton or some other respected company. They claim that your computer has reported a virus, and that they can remove it remotely, for free.
If you agree, they'll start a remote session on your computer, which you'll need to approve by clicking on a button. They'll appear to be removing a virus, meanwhile they'll be planting a key-logger or other malware on your computer that will give them full access to your machine, including all your stored passwords, etc.
Please, if someone calls you with a virus story, don't agree to let them "fix" it.