program that attaches itself to email
sent by the host computer. AKA: Trojan.Happy99, I-Worm.Happy, W32.Ska, and Happy00.
Do not open.
If, like me, you aren't paying attention and open it by accident, don't worry. It doesn't do anything terrible to your computer. It merely shows a firework display to disguise its installation and begins sending itself to everyone else you email, eliciting nasty "why'd you send me a virus" type responses.
For help removing it, go to www.sarc.com/avcenter/venc/data/happy99.worm.html.