MSBlast(ed!)
I installed XP Home yesterday. Brought it up to speed with all the proper drivers (Compaq's are a bitch that way. Every little single component needs it's own special driver that they obviously don't supply to Microsoft to include), got it connected to the network, and then went back to my Mac.
Got in this morning, found I can't connect to the net. Why? that wonder msblast worm.
Lovely.
Ah well, at least several other of my team members had gotten it, so it was a simple matter of getting them to copy the fix for me. Took me all of 3 minutes to fix.