Could be any number of things, up to and including a virus/worm trying to spread itself, or using your PC for sending spam emails.
Download, install, and run
Spybot - Search and Destroy , and make sure you have antivirus software that is kept current. New viruses are discovered every day, and most antivirus software had updates weekly.
Make sure you have all the critical updates from Microsoft. Most worms exploit vulnerabilities in Microsoft software, one way to avoid them is to fix the holes.
My personal opinion is never use Outlook. Lots of free mail clients out there, without the "features" that make outlook so enticing to bug-writing-jackasses.
Tom
Tom