This one appears to be genuine. I don't usually get sent viruses but I've received this one three times in the last three days and it almost fooled me. Escaped thanks to this warning from the head honcho of Liberal Democrats Online:

Just to warn you all that a new virus has hit the scene (I've just received it, and it might be from one of you).
The email it comes in describes itself as a Microsoft Internet Security Update, but it isn't. Microsoft don't send out updates by email. In no circumstances run the attachment to the email. Delete it immediately.
Details at
securityresponse.symantec.com/avcenter/venc/data/w32.gibe@mm.html