unfamiliar login on facebook

I went to facebook and noticed that Safari was already running -- I could have sworn I quit it before I left (and I put the mac on sleep) .. When I went to facebook I saw a strange email login address in the login bar. Has this sort of thing happened to anyone else and can you offer any explanations?

Do I have some sort of virus and if so (and in any event) is there any software out there that can search for viruses in any application?

Usually this would mean that somebody used your computer to log into facebook. Does anybody in your house / office have access to your computer? Does it have a password on it? did you leave it where anybody could walk up to it? etc. If so, find out who's e-mail address that is and kindly ask them to ask permission before they use your computer again.

And no you do not have a virus or anything as there are none, and there will be a simple explanation. On AV and malware read this:-

Mac OS X anti-virus software: More trouble than it's worth? | MacFixIt - CNET Reviews