Could be a number of things... some things to try....
Windows installations typically leave some kind of log behind that you can use to see why the install failed. Maybe something was logged to this file before it rebooted. I found
this list of log files that you may check.
Also, if you system is just spontaniously rebooting, it could be because you have memory problems. You should definately run a memory test on your RAM before you go and return the drive.