Sorry, if you do not have this key into your registry (HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\VirtualDeviceDrivers) in a XP system, you do not have avast! correctly installed if I’m not wrong.
If you uninstall avast and install it again, won’t this key appear? If not, do you want I post it for you here in forums so you will be able to import it to your Registry?