XP Hanging on Shut Down

PLEASE HELP !!!

Hi, I have a machine with windows XP Pro.

It hangs on shut down. If you restart the computer & go to shut it down it will do so straight away. but if you leave it sitting idle for more than 15 minutes with or without a screen saver running it hangs on shut down, if you are playing a game for an hour & go to shut down it hangs.
I have set power options to never & un-ticked hibernate in the power options.
I have installed XP shutdown Patch & Autoend Patch (supposed to force shut down of any running programs in the background) with no success. I am going to replace power supply next. does anyone know of a solution or can you recommend another thing to try to solve this as it starting to drive me nuts.