When you are having the post screen, try to slow it down (enter to BIOS or any startup menu before Windows has started to boot) and then hold power button pressed for ~20 seconds. The computer turns itself off.
You can shut it down this way also being in Windows, but then it could report about failure and you could have some disk errors also if disk writing procedure was in progress.
Try to look for newer BIOS version at the support downloads site.
Try to unplug any USB devices, which you have connected while rebooting (except keyboard and mouse).