Although viruses are relatively common today, just as common are hoaxes. Also, due to the way the media often reports on virus outbreaks, user concerns often lead to benign computer problems being misdiagnosed as viruses. To learn more about what a virus is, read through Symantec's definition site and other related pages Denial of Service.
In general be aware, but be skeptical. There are many virus hoaxes (and other scams) out there, most typically distributed via email. If you receive email that sounds too good to be true or warns you about a virus, check for more information on Symantec's Hoax site and read Microsoft's Security notice information.
Unless you know the sender of the email AND that they intended to send you the message you received, DO NOT open attachments you receive in email.
If you still suspect you have a virus update your virus software's definition files and run a full scan of your computer. If a virus is found visit your virus company's site to read up on the virus to see how you can protect yourself from getting infected again. Or use Symantec's Security & Virus Check and get detailed information on Symantec's virus information page.
Click here for tips for avoiding computer worms (one type of virus).