Klez virus - ALWAYS uses address books only?

I just got a notice in my Inbox that said "The email you sent had a virus attached to it!
The University of Vermont's email anti-virus software has deleted the
infected attachment from your message. ..."

I know this was most probably sent from someone else's computer that has my e-mail address and the one for the person at the Univ. of Vermont along with the virus, but is that always true? Does it always act like it's sending from someone else's address (in other words, IF I became infected, would ANY of the e-mails it sends from my computer look like they're coming from me)? Also, does Klez only pick from an address book? Or does it grab some addresses from any other places on the infected computer like other e-mails in the Inbox or files stored somewhere else?

I ask because I'd like to make sure this really wasn't sent from me. The address it was sent to isn't in my address book and doesn't look familiar, but without searching through all the headers in all the e-mails in my Outlook Express, I don't know for sure that it's not listed in there somewhere.

Thanks!