(... being a selection of words that might get Googled, to help future sufferers ...)
I have spent several days doing battle with one of our computers to try and resolve an issue that arose.
It started with the PC rebooting itself on several occasions. Eventually, it would get as far as the "black screen", but then reboot before it reached the "blue screen" with user names on it.
First thoughts - perhaps a power supply problem (it was only 250 watts) or a motherboard problem. So I bought a new box with 400W PSU and a new motherboard/processor from Maplin, attached the same peripherals ... and the same thing happened.
Okay, so perhaps the installation of Windows was corrupted, by all these reboots before it was running properly. The HDD still worked, so I tucked it into another computer, and copied all the important files onto it, then I tried to reinstall Windows. That was fine to start with, until I installed SP2 - a bit of a battle even getting that far, because doing that much requires reducing the security on the network at home so that the original Windows XP can access the Internet, downloading and installing about 50 incremental updates, and waiting half an hour for SP2 to install. And then ... same symptoms.
Tried it with a new HDD. Same problem. Tried disconnecting everything, to see if it was a hardware fault somewhere else. Same problem.
Eventually, following one more trawl through Google results, I tried it with a different copy of Windows XP - one that had come bundled with a different computer. The usual hours spent updating it - and hey presto! It worked!
So what was going on? I think what had happened was that one of the drivers that was installed with the original installation violated a part of SP2 called Data Execution Prevention, and I had "installed the 32-bit version of Windows XP SP2 on a computer that supports hardware-enforced data execution prevention (DEP)." This may have been the DVD decoder that came with one of the bundled software packages - if so, it wasn't the mpegport.sys one referred to in the Microsoft bulletin. So with the same hardware but a different software package bundled from a different manufacturer, it worked.
How come the problem arose initially? I suspect that this wasn't the actual problem on the original computer, but that there may well have been a PSU problem - and it's just that this other problem became apparent when I tried to run it elsewhere.
Anyway, if you found this by googling because you are having a similar problem, all the best ....