I have seen a few threads about Windows restarting instead of shutting down. Although well documented there are not many solutions.
I had the same problem and none of the advice currently available seemed to work. However after some trial and error I discovered it was down to one of my USB ports. I have a four port PCI USB card installed. For some reason if anything is installed via one particular USB port it causes Windows to restart instead of shutting down. Not sure why but I have kept that port free and everything now works fine.
Hopefully this may assist others who are experiencing similar problems.