Win32.Sality.A is a polymorphic virus that infects Win32 PE executable files. It also contains trojan components. Win32/Sality has been known to be downloaded by variants of the Win32/Bagle family.
This was brought in on a usb stick and quickly infected the network due to some incorrect permissions on certain shares, this is likely due to the progression from NT to Server 2003.
A number of years ago just as we were staring to roll out windows xp on the desktop machine we were infected by a staff laptop that brought in the Sasser virus, this was able to be tracked and keep on top of, plus we only had one lab at the time running xp and some office computers, the rest ran windows 98 and were unaffected.