Can't Extend Desktop WinXP/32, tried 3 different video cards and 4 different monitors

[Guest PacificEnvironmental]

One of our staff member's system went down -- since we all run identical systems (P8H77M-CSM, i5-3570k, 8GB DDR3 1600), I just gave her one off my test bench.

 

 

Sometime between connecting the system, powering it on, and connecting her 2nd monitor, XP decided that it would no longer like to extend the desktop to the second display -- the machine was working fine in extended mode prior.

 

 

Thinking perhaps the integrated graphics (Intel HD4000) or DVI connector failed, I installed an external graphics card (nVidia 8400GS) and disabled integrated graphics at the BIOS. Windows still refuses to extend the desktop.

 

 

Thinking perhaps, by chance, the monitor or cables might be the culprit, I swapped hers with two known good ones (mine, 2xDell S2240M), and rebooted. Windows still refuses to extend the desktop.

 

 

Thinking perhaps, by some act of God, that the nVidia card died, I replaced it with a known good video card (ATI Radeon 5450) and installed the latest Catalyst drivers. Windows still refuses to extend the desktop.

 

 

 

So, 4 monitors and 3 video cards later, all of which I now suspect are good, Windows still refuses to extend the desktop. Either this is an impossibly rare set of coincidences or the issue isn't hardware or driver related.

 

 

What part of Windows would cause this problem and how do I correct it?

 

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