Spent a few hours this evening trying to figure out why Windows XP wouldn't boot up on my second disk partition. It would just get to loading the agp440.sys driver and hang. This was completely baffling as it had been working fine and I'm sure I hadn't installed anything new the last time I had used it. The agp440 driver is related to the video card but this is a red herring since after this driver, it loads ACPI stuff. From reading around the 'net it looks like this hang indicates a hardware error somewhere.
I tried getting more info by enabling boot logging but I couldn't get it working. Disabling the agp440 driver from the Recovery Console didn't work either and confirmed that it was probably a hardware issue.
So I started unplugging things and low and behold it booted properly after I unplugged the mouse! Weird. It's a USB mouse so maybe there was an issue with USB? Sure enough, I found this patch, installed it and it's working fine now.