Have you tried restarting your computer since it started happening?
I've never used Entourage, so I can't help you there. I've never had that happen with Adium, but sometimes Safari unexpectedly quits on me (not that often though) like that. If it's happening every time you open Adium, maybe try uninstalling it then reinstalling it. I don't know if that will work or not.
I know Mac's aren't supposed to get viruses, but do you have antivirus software installed? I have a free one called ClamXav on my MacBook. I only run it maybe once every few months, but I haven't found a virus yet.