Question : Problem: No longer able to extend desktop to secondary???

Here's a puzzling one:
      a client has a stand alone PC that has a Radeon X1050 (1x VGA output and 1x DVI output). They are using a 17" LCD monitor as primary monitor, and a projector as secondary.  They extend desktop to the projector so that their software can use the secondary to display a seperate output to what was on the 17" LCD (basically the same idea as using Presenter View on Powerpoint '03 or '07)
      This has been working for a year or more until of a sudden, about 4 months ago, XP started rejecting the secondary. Such that if I go to display properties and enable monitor 2, click 'Extend Desktop' and then click apply nothing happens! It just goes back to appearing as though there is no secondary available. The projector clones the primary monitor just fine, but I cannot get it to extend desktop for anything.
      Have swapped out the video card for a brand new - same problem. Swapped the outputs over - same problem. No software changes have been made on the PC except windows updates. Definitely got all the latest drivers, even tried rolling back drivers as far back as possible.

   I'm running out of ideas, so thought I would ask to see if anyone has come across this problem with XP before. Can anyone think of anything?

Answer : Problem: No longer able to extend desktop to secondary???

you might see that an automatic windows update was performed on this PC recently.  There were several windows updates that were downgrades, and they do not correctly support dual displays, especially when it concerns "projectors" that do not register as a "safe" device in the new windows device model.