* Win32/Sality.NAR (NOD32 detection). This is a polymorphic file infector which searches local and network drives for files with the .exe extension and infects them by adding a new section that contains the viruscode.
It also copies itself into the root folders of removable drives using a random filename and creates an autorun.inf file to make sure it runs whenever it is inserted into another computer. It also disables most AV scanners by terminating their services/processes, disables Taskmanager, disables Regedit and much more to prevent it being detected or disinfected.
In this case, the user had an USB flashdrive and used it to transfer removal tools etc in order to remove this infection, since no scanners would work. What happened was, since this virus also spreads via removable media, his USB flashdrive became infected > result > His other computer was infected as well!
*W32/AutoRun-OY - This one also spreads via removable drives. This computer is used at home and every user has its own account. Mom, dad, son and daughter. Son loves to play games, but also loves to download games + cracks via illegal resources.
See here how to protect your data
This appears to be a common problem nowadays - that's why it is so important to prevent spreading similar infections by disabling Autorun.
To disable autorun, please read the following tutorials:
http://www.howtogeek.com/howto/windows/disable-autoplay-of-audio-cds-and-usb-drives/ (applies for XP Pro since XP Home has no gpedit.msc present)
http://www.engadget.com/2004/06/29/how-to-tuesday-disable-autorun-on-windows/ (aplies for XP Home. Same can be used for XP Pro)
http://www.howtogeek.com/howto/windows-vista/disable-autoplay-in-windows-vista/ (applies for Vista)
Some malware removal tools already disable Autorun by default. Don't complain about this. This is an extra security measure and you should have it disabled. If you really want to enable this again - then it's your own responsibility. Don't complain afterwards if you get infected and are responsible for infecting a lot of other computers as well.
Update: Extra instructions to disable autorun (by US CERT) can be found here.