Well I've Seen Some AV such az Norton and AVG that uses VxD files to detect viruse type files.
Well it's very hard to kill these AVs. a DOS Screen will appear when a Trojan/worm/virus is Detected and Alarms!
Who knows How these VxD's are loaded and how can we kill their process ?
Hint: if a trojan uses the same approach it will hard to clean it ! and it would be a new way for trojans start up!
( I thought that VxD's are just for Hardwares! )