I’ve been getting random IM’s from infrequent contacts on Windows Live Messenger (MSN). These all have a weird domains or “viewprofile.php”-style links, which is usually a good indicator that your system is presently compromised with a trojan or worm. Run a virus scan if you’ve been getting complaints. Stinger will probably even take care of things.
Since I run a MacBook using That Damned Quacking Duck for instant messaging, my own smug sense of superiority is enough to prevent infection.