on my production machine; the thing I do my release builds on.
I tried the obvious things, then pulled the drive and mounted it on a machine in the lab as a second SCSI drive. My data partition was untouched. The C drive was missing the entire "Documents and Settings" directory; ie. no registry any more. Truely FUBAR. I saved my data without incident and the miserable box of bits is probably still re-imaging itself now (we use a LAN based installation system to load a standard image.)
It seems sort of a coincidence. The IT shitweasles have been pushing every patch and anti-virus they can collect onto my machine and no virus has been detected. All our machines have been slowing down though. I can't really say who wrecked my machine. Could have been all the crap pushed by IT; could have been a virus that is undetected; could have been bitrot, as I haven't reloaded the machine for about 15 months. It's probably something worth keeping track of.
Hope you didn't lose anything valuable. Good luck.