Some thoughts and relevant info:
I only have one drive, and it's connected to the motherboard via a single ATA cable, not IDE. I've had an issue or two with the SATA drivers in the past, but the windows installer seems to be loading the SATA drivers, so it seems strange to me that there might be an issue with the motherboard detecting the hard drive. Also, I previously slipstreamed the drivers into my XP install disk.
The problem didn't arise after installing new hardware or software. The computer froze while running a game, and it hung on startup when I rebooted it manually. It occurred to me that the issue might have something to do with the AGP card, but I don't understand why that would interfere with startup, especially since I'm having no display issues.