Two separate bugs, both being exploited.
Trillian has released an update to fix things up. Get it here:
http://www.ceruleanstudios.com/downloads/
No fix for WinAmp yet, best advice is don't open untrusted MP4 files. MP3's are safe.
More info about these bugs at
Trillian: http://labs.idefense.com/intelligence/vulnerabilities/display.php?id=522
WinAmp: http://secunia.com/advisories/25089/
