Bob, it's a problem on the VDD registry key like RejZoR said:
Anyway it will be fixed in upcoming avast! 4.5
I can't help more as the user is using a 'clean' XP SP2 installation.
In the past, problems like this were related to Norton antivirus installed before avast.
The user must check what he has in this Registry keys:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\VirtualDeviceDrivers\\VDD
HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\VirtualDeviceDrivers\\VDD
HKEY_LOCAL_MACHINE\SYSTEM\ControlSet002\Control\VirtualDeviceDrivers\\VDD
HKEY_LOCAL_MACHINE\SYSTEM\ControlSet003\Control\VirtualDeviceDrivers\\VDD