Pretty insidious...it shows up as a message from the network administrator as detailed here.

A new Windows mass-mailing virus, which disguises itself as a file sent by a computer user's network administrator, began infecting systems on Friday and quickly rose to the top of the virus charts on Monday.

The worm, attempts to exploit a vulnerability in Internet Explorer that allows a script to be executed by an infected computer. The worm then tries to use that script to mass e-mail itself, potentially clogging mail servers or slowing down networks, according to antivirus company Symantec.

The worm spread widely on Friday, with the bulk of messages affecting computers in the United States, according to data from UK e-mail provider MessageLabs. The firm bases its figures on the number of copies of the worm stopped by its customers' e-mail servers around the world. On Friday, MessageLabs stopped more than 25,000 copies of Mimail, and after an expected drop-off over the weekend, collected more than 10,000 copies by 13:00 BST on Monday. These figures put Mimail at the top of MessageLabs' virus charts, ahead of Klez and Yaha.