Nathan,
It's probably one of the Outlook worms. Some of them read the infected machine's address list and send mail to users on the list. In addtion, some of them send mail as another user from the address list. Usually if there are attachments they are the worm trying to spread itself.
On Windows systems .SCR files are screen saver files. These are executables and can cause much grief. IIRC you are running a Mac, so that won't be an issue.
hth, mike