Eu encontrei isso. Parece lógico. O crédito vai para o Fleece por este tópico ....
My solution to this problem was to set this registry key carefully to value of "0" (zero)...
HKEY_LOCAL_MACHINE\Software\Microsoft\TMM\UseIviewHelper
..., next fully uninstall the NVIDIA drivers and control software, and reboot with only the primary and desired #1 monitor connected. That reset the #2 monitor as #1 monitor. Then, reinstall the NVIDIA drivers and control software and setup second monitor. Problem solved.
Credit goes to this external forum post... Dual Display Numbering in Vista
Isso presume que você esteja usando drivers NVIDIA. Se esse não for o caso, " do Microsoft Answer Numeração Dual Display no Vista "thread tem quatro outros métodos. Confira.