Since you don't get anything written to the disk, I would guess it's related to that. I myself experienced similar crashes on S2k8R2 recently, where the computer would stop doing anything for a minute or so before halting, particularly during heavy disk throughput like streaming a video. The problem in that case turned out to be a damaged SATA cable, but I guess a damaged disk or SATA controller are equally possible.
Try to check what the latest entry in the system log (or system event log? I'm not currently on a windows system, can't check.) before the reboot is, if there is nothing it's probably a similar problem. You could also disable automatic restart, so you might get a bluescreen explaining what caused the shutdown.
Finally, to make sure it is a disk related problem, you could try stress-testing the drive. I'm sure there are a lot of windows based benchmark tools for this, but I usually just boot a linux live disk and
dd if=/dev/sda of=/dev/null bs=1M and watch the system log for errors. If this provokes errors or hanging, you can be quite sure the problem is related to your disk.