Thank you for your answer. Since this is a complex problem I will give you a bonus if you can help me to sort it out.
Your suggestion unfortunately didn't work. When I attach the third monitor the PC shifts my two previous screens to the left, so what was the central screen becomes the left-hand screen (i.e. the new third monitor attached by USB adaptor) and what was the right-hand screen becomes the central one. Meanwhile the left (third) monitor resolutely remains dark. This means I can't adjust any of the settings, because everything disappears into the dark 3rd monitor, which the PC believes is my main monitor. In other words, if you open the display properties box it opens in the dark 3rd monitor. I have tried moving around the monitor icons in the display properties box, but nothing works.
Thanks, XXXXX XXXXX do I find "set as default"?
Also, as I said in my first post I'm trying to connect using a USB display adaptor. My graphics card is a dual card.
I've tried all of this but it doesn't work. As soon as I connect the 3rd monitor the computer thinks it's monitor 1, no matter what I have done with the icons before. Of course, if the Dell monitor would wake up it would't matter as I could manipulate the display properties box on the 3rd monitor. But instead it remains stuck in darkness, even though the PC obviously thinks it's working.
Part of the problem is therefore the fact that the Dell monitor will not respond. I have another monitor, an ACER, where it DOES work, but I prefer the Dell because of the image quality.
End of the road?
I have found a workaround for the problem as follows:
1. Turn on PC
2. Take out VGA cable of 2nd monitor from dual graphics card and plug in 3rd monitor instead. After a few seconds, swap back and plug the 3rd monitor into the USB monitor adaptor
3. The 3rd monitor has now been "woken up" and remains alive after being plugged into the adaptor. The position, resolution etc. can then be controlled using the normal control panel options in XP.
Having done this a few times the 3rd monitor now seems to realize it's supposed to be awake and comes on even when plugged into the adaptor at start-up, in other words I no longer have to switch the plugs around.
So the problem comes down to getting the DELL monitor to get out of sleep mode and/or getting the PC to expect that a third monitor is attached.
I think this is the problem solved now. Do you have anything you want to add?