If you're having troubles getting it to power on and stay running, open the side and turn a fan on to help cool it down. If it's a heat issue, this should help. Then you'll want to download speedfan as mentioned above.
My desk PC started shutting down randomly and finally every 5 minutes. I'm using Windows XP. My solution: Aftrer removing an air shroud that was blocking my view of the CPU, I found dust had covered the CPU fan, impeding the air flow and vacuumed it off. The unit has now been running for 2 days steady and no further problem.