I'm encountering a very peculiar problem, I think.

Now, I've seen a lot of people whose issues are superficially similar to mine (black screen in XP after splash screen; Safe Mode boot hangs on mup.sys), but the way I've seemingly fixed the issue makes no sense to me.

I recently upgraded my video card, so I thought that might have caused an issue, but, for some reason, I decided to check my RAM before I did anything else.

I was running two 2GB DIMMs, and I don't recall running into any problems before. I found that I can boot into XP perfectly if I remove one of the DIMMs (no matter which one it is). But with two installed, it always hangs after the splash screen. I'm dual-booting Windows 7, and that boots just fine no matter what. Furthermore, XP will also boot if one 1GB DIMM and one 2GB DIMM are installed. I only encounter an issue when I'm using 4GB of RAM.

I know all about 32-bit addressing limitations, but I don't see why this would cause boot issues. Also, like I said before, I don't recall ever having problems booting into XP, even with 4GB of RAM installed. (I can't even tell you when the last time I logged into XP was, though, since I've been running Windows 7 exclusively for a while).

Does anybody have any ideas about what could be causing the issue? I'd really rather run with 4GB, since I'm using the 64-bit Windows 7.

Very strange indeed. You could have stumbled upon an obscure bug in Windows XP. Do you have two pairs of RAM slots? Have you tried shifting the DIMMs over? Often times when I face problems like this I'll try to find a work around instead of a solution to the problem. If I was doing the troubleshooting I'd try updating the BIOS and swapping modules for another pair of 2GB DIMMs. To make a complete guess, you could have a slight imperfection with your hardware causing data corruption that only occurs with XP. The OS likely uses different memory addresses during bootup and completely different code for the bootup procedure (vs. Win7). Also, did your PC boot normally before you installed the new video drivers and quit working after you installed the new drivers?