If you're getting odd emails from people who seem to think that you've sent them a virus (when you haven't emailed them anything at all), it may be the Klez virus or one of its many cousins, siblings, illegitimate children etc. at work.
When these viruses infect a computer, they send emails (with themselves attached in a variety of disguises) to addresses in your email or other address books on your computer.
They use other addresses from the address book as the return address on the emails.
In other words, Bob gets the virus. Bob has Carol, Ted, Alice and you in his email address book.
The virus finds all the addresses and sends friendly-sounding email with bad stuff attached to Carol, Ted and Alice, but the emails look like they came from YOU, Bob.
Ted doesn't have a virus checker program and foolishly launches the attachment, infecting his own computer. And maybe blames you for it when he finds out.
Carol has a good anti-virus program, which flags the email "from you" as a nastygram. You get an angry email from Carol about it.
Alice's moved onto a different company so the email addressed to her bounces. Back to you.
So now you've got this in-box full of unhappy people and bogus bounce messages and there's not really much you can do about it except to send anyone who complains a link to this page so they can at least learn what's going on.
And of course, you'll want to make sure you don't have the virus yourself.
Symantec's Site has lots more info on Klez, along with instructions and tools for getting rid of it once you have it.