XP shutdown (doesn’t complete)

Hi,
I have XP Pro on two different computers (each has its own original CD) and both sometimes perhaps 1 out of 10 shutdowns won’t shutdown completely and turn off everything within the tower. You can hear the tower click but the neon lights within and fan still work within. The other computer does the same (Except the neon lights as it lacks that). Pressing the reset button automatically shuts it completely down instead of rebooting. Holding the power button definitely shuts it down. It just states that Windows is shutting down (beyond logging off screen). This is pretty much the screen where it is okay to manually shutdown if necessary. Any ideas on why this could be happening?