Go into your mailbox and look for a message with suspicious attachments. Delete that message and attachment, then compact your mailbox (this step is important, because it will ensure that the virus is actually removed from the mailbox after you've deleted the attachment). That should kill the virus, but scan the mailbox one more time just to make sure.
You haven't become infected by the virus, but as others have noted, you're a "carrier". In other words, there is a file on your disk with a virus in it, but that virus cannot take root and infect your machine. As long as you don't deliberately send that message to PC users, you won't be able to infect them either.