Did you check the options in your shutdown window and make sure the option 'to shutdown' is selected?
It has 4 (four) options. If 'restart' or 'restart in MS-DOS' are checked, when you hit OK to initialize shutdown your computer will reset and restart.
The computer remembers your last selection and any successive attempts to shutdown either hard booting or using the start>shutdown window will cause rebooting.
You must manually change the option in the window to 'shutdown' from any other of the selections; then select "OK".
I did that myself when I first got my PC. I was a confirmed MAC user - graphic artist.