Mike, what video card are you using? Have you considered loading a different video driver from the default? Most often, such issues as you describe are hardware related and it does not mean bad hardware, but perhaps a faulty driver. Anyway, give this command a try. Open up a terminal session and type:
Gateway NV52 running openSUSE 11.3, GRUB, (and Windows 7)
When I select SHUTDOWN, SUSE stops, then the computer re-starts. When I shutdown from Windows, the computer behaves and does not restart.
And report back if this works (shuts down your computer) and what Video chipset you are using. Also, 32 or 64 bit openSUSE and desktop (GNOME or KDE) used.
shutdown -r now