Reply #4: Not necessarily an infection.

[trotsky]

Many viruses, when they infect someone, take that person's e-mail address book and then craft a bogus message using one of the addresses as the "TO:", and another as the "FROM:". Your address was likely picked at random to be FROM, and since your address is legitimate, when the virus message can't reach a recipient, you will get the notice.

(Some viruses also scan a user's web cache files and other locations looking for e-mail addresses. So your address can be picked at random even if it's just posted on a web page somewhere.)