This might be due to corrupt windows registry, use a registry cleaner program to boost your PC performance:
Download: http://download.cnet.com/RegInOut/3000-2094_4-10965219.html?part=dl-10053761&subj=dl&tag=button
Ah NO ^ you don't want to do this. How will a corrupt registry cause a PC to start after 5 hours?
What is the issue you are having? Sounds like you did not word it right. Does the PC RE-start after 5 hours? During gaming? When just sitting there? What is going on exactly?