If the additional external screen causes the computer to turn flash the screen this reg key can disable the initialize:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\TMM
navn: PollingInterval
type: REG_DWORD
data: 0xffffffff
efc0b8d6-21b4-4c9c-8b37-e4fa9b1f6bea|0|.0
Windows Vista