No signal on DVI socket (Dual / multi monitor feature)

I had an HP Media Center m587 with two Dell flat panels connected to it (1702FP) with a NVidia FX5200 graphics card.  For about six months it worked fine with two monitors (one connected to VGA and the other DVI).  Then in September the DVI stopped working and therefore only one screen would work (ability to add second monitor was not there).  After many calls to HP it was sent for repair and after 10 weeks they said it was a generic problem with my model and offered me a new one.  Full system resets etc were all done but did not work.

Today I have installed the new HP Media Center m1190 (Windows Media Center 2005 with with ATI Radeon X600 Pro and it also has the same problem in that the DVI signal will not work (alone or in conjunction with a second monitor on the VGA socket).

I have tried:

Switching the monitors around and using a third flat panel
Trying different leads
Reinstalling the drivers for graphics card and monitor
Changing the BIOS
Using a DVI to VGA convertor (which works but doesn't explain why the DVI doesn't work)
Using the switch on the front of the panel to change the input (message comes up saying it's in sleep mode)

I have read through many forums and tried various solutions but it nothing seems to fix it.  The option to add a second monitor is there but it doesn't do anything.  It seems a lot of people are having this problem but none of the fixes other people have tried has worked for me.
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Hmmm....September was around the release of SP2.  Is SP2 installed on your computer?  Try this, open a command box (start->run->command) and press ALT/ENTER combo for full screen.  Did that wake it up?  If so, try downgrading to catalyst 3.19 driver.
err...that should be "catalyst 3.10"
After further research I suggest you go back even further to catalyst 3.7
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dvsukAuthor Commented:
Tried that but nothing on the DVI.  

Media Center 2005 has SP2 built in - no option to remove it.  Having looked on all the forums, it seems lots of people started getting these problems recently so it could be something to do with SP2 - although the problems on my old pc didn't occur immediately after I installed SP2, plus a system recovery didn't fix the problem on it either (that had SP1).
What happens if you disconnect the analog monitor and only connect the DVI monitor?  Can you see the POST?  Can you see Windows start to boot and then disappear?
dvsukAuthor Commented:
When the PC starts up nothing happens on the monitor connected with to the DVI socket - no POST or startup.  On my previous computer, both screens would switch on and show Windows start up, the secondary monitor would switch off at the login screen and would return when once you had logged in.

The card can't be faulty as this is the third card I have used with this exact same problem.  There is a VGA signal coming out of the DVI socket but not a digital signal.  There has to be something wrong with the motherboard, BIOS or Windows.  This is very irritating!

I tried installing the latest drivers from ATI (although these weren't installed by HP so I uninstalled them when it didn't work).  I haven't tried installing the 3.7 or the 3.10 as you suggested above as you comment seemed to imply I should do this if the screen worked.  Also, not sure where to get older drivers/software from.
Apparently some monitors are sending an incorrect EDID and the video card thinks the monitor is analog - hence no DVI.  This thread has a possible solution (warning - pretty technical fix).

See the 3rd post from the bottom.

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dvsukAuthor Commented:
Hey thanks for the information.  All fixed now.  

Problem appeared to be related to the EDID as you suggested.  I read through the forum and found a simple fixed which is available on the Dell Website.  Basically you need to run a program in DOS with the monitor connected via the DVI which awakes the monitor out of sleep mode.  I didn't figure it was the monitors as I had tried a number of monitors but they were all the same model.

The fix is for the monitor and only works with NVidia graphics cards.  I had to connect my monitors to my old machine and run the software as it didn't work when using my new machine which has an ATI graphics card.  

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