Computer restarts after returning from sleep mode

Hi, I have this problem with sleep mode. I am using Windows Vista Home
Premium SP1.

First, I put the computer in sleep mode manually by going to start and then
clicking on sleep button. After a few hours, I come back and wake it up by
clicking my mouse or pressing a key on my keyboard. It wakes up and my
monitor turns on showing the desktop but after a few seconds it shows the
blue screen and restarts. This happens everytime I put the computer in sleep
mode for a few hours or more. I don't understand why this is happening.

I already have the latest drivers installed for my soundcard and video card
which is Catalyst 8.10.

Can anyone help?