Problem logging off the computer

When logging off of my computer and I leave it alone for a while I come back and the computer is frozen. I have my power managemnt settings set to turn off the monitor after 10 minutes of it not being used. When trying to get the computer to come back I would move the mouse around but this wasn't working. When turning on the monitor the screen was black and there was nothing to be seen. I have Windows XP Professional and I hope someone can help me. Thanx