Check out the various keys and folders in HKEY_CURRENT_CONFIG\System\CurrentControlSet\Control\Video. Specifically
HKEY_CURRENT_CONFIG\System\CurrentControlSet\Control\Video
Specifically in the inner most MonXXXXXXXinnermost MonXXXXXXX
sub-folders should be Attach.RelativeXAttach.RelativeX
and Attach.RelativeYAttach.RelativeY
values which are the monitors' location in relation to pixel 0,0.
Strangely when I check a Windows 7 machine here with two monitors, it doesn't seem to show the second monitor. But my XP machine shows all three of mine, so your mileage may vary. :)
If nothing else it's a place to start. ;)
HTH.!