On two identical machines, I typically cannot restart Windows from the XP64 partition. Windows closes normally, and the machine goes to the point at which it should start the POST. The monitor goes to sleep mode, and the keyboard and mouse are dead. There it remains. I then kill the power and can restart okay. If I shut down from 64, the machine shuts down okay. I can restart fine from the XP32 partition. I'm using a Tyan 2895 with current BIOS, two dual core Opterons, 8GB RAM, MSI video card. The 32/64 system is on an Ultra SCSI HD on the MB controller.

I had a similar problem a while ago, it turned out to be a bad driver for the sata controller. Make sure you have the latest drivers for your SCSI controller and everything else too.