Current problem: Gaobot worm. It's out there on our network somewhere, probably brought in on a laptop like Nachia was last time.
Nowadays we have to update the Operating System patches and Antivirus Definitions offline before plugging into the network, or Gaobot disables McAfee ViruScan and you have to reimage the PC from scratch.
McAfee says it can clear this worm off if you enable scanning of compressed files, but in practise it doesn't seem to be able to. I eben tried slaving an infected HDD off a clean HDD and scanning from the uninfected OS. Still no dice.
If anyone out there knows how to kill Gaobot on an infected PC, I'd be interested to hear about it.

Our site has had many dozens of Fujitsu hard drives break down. It appears they run WinNT OK, but Win2000 makes them break in 12-15 months. The problem is well known, and is to do with the HDD BIOS which is on a chip on the backboard on the HDD itself. In theory, you can revive a dud HDD by swapping the backboard though I have no examples of this having been done.
This has been one of the most time consuming problems on my site during the last couple of years.