I am having the same issue. I set the resolution to 1024x768 32bit color. And after sitting at the desktop for about 5 seconds, the screen switches to the native resolution of the LCD which is 1280x1024. I work in IT at a community college and this is very aggrevating for my users (and myself). This is only happening on the latest models (Dell Optiplex 760) with Intel graphics. We are using the latest video drivers from Dell for that model and it is fine on most of the machines, but in every computer lab of 25, there are a handful (~5) that refuse to stay at the lower resolution. I tried the latest drivers straight from Intel (didn't help) and I also updated two of the problematic machines today to the latest BIOS which mentioned something about an update to the Video BIOS. Still no help. I reimaged a problem machine and it exhibits the same behaviour. The only time I can get those machines to not change resolution is when I completely remove all traces of any Intel driver and let Windows use its generic display driver (then everything is fine). But for me, this is not an option. Anyone have any ideas or suggestions?
I have 7 labs of the dell optiplex 760s which we just got over the summer. There were a few that wouldn't display properly with our ghost image. I was able to go into safe mode and when I removed the intel drivers the machines worked fine. Since we needed them to work with ghost images, dell replaced the motherboards. They work fine now. I am getting reports from users that the resolution is changing on a few machines similar to your issue. I haven't re-ghosted them but I suspect the mother boards are not working properly and it won't help.
If you want to know if you have a hardware issue or a software issue, is probably best to cross test with a different O/S For example VISTA.
If it works fine in Vista clearly you have a software issue. What drives version you have installed?
Order on which you installed the drivers? For example: Chipset, Audio then video?
What service pack does your XP have? Is 32 bit or 64bit?