Antivirus software's job is to pretect you from bad things. They interrupt evey call to disk or memory and see if anything bad is happening...that is a good thing because it makes you machine safer, it is a bad thing becuase some things do not need to be scanned. Antivirus software differ in how they allow exclusions, some do processes, some file extension, some directories.
You can improve performance on Virtual Server by excluding the following process from being scanned
VSSRVC.EXE
Also exclude the root directory that contain your Virtual machines and configuration files
Also exclude the following files extensions
VHD, VSV, ISO, VUD, VFD, and VMC

