It's probably the Klez worm or another version and it copies address books then sends the virus to others with your email address even though you have no virus.

ie: John Doe emails 5 ladies.  He now has all 5 ladies in his address book.  John Doe gets an email and downloads the virus thinking it's from a friend.  Immediately this virus copies his address book and sends the virus to each person in his address book with new email address making you think someone different just sent you a virus.  

Make sense?  

"The worm has the ability to spoof the From: field (often set to an address found on the victim's machine)"

So... if you're receiving emails from friends, this does not mean they have a virus on their machine and it certainly doesn't mean that friend sent you a virus.

For more info, go read about the Klez worm on your antivirus software site.

I use McAfee, but there are others around.

Download a free anti virus program and test your system for virus.  I run my anti virus (McAfee) in the background continually along with a firewall (McAfee) and encryption (PGP).  Not a bad idea if you're worried about who might get or have access to your system.

Good Luck
Misty

Actually in addition to the address book most newer viruses including Klez will search your hard drive for e-mail addresses.  They harvest them from cached web pages and any other document that they can read that contains e-mail addresses.  This is why you can still receive virus infected mail and be spoofed as a sender even if your not on anyone's address book.  Stay safe, don't open it if your not expecting it and visit http://windowsupdate.microsoft.com and ensure you have all critical updates installed.