If you're uncomfortable with preforming any of these steps, I suggest taking it to a PC repair tech... Examine the Event Viewer under the Administrative Tools... Click - Start > Run > and type CONTROL ADMINTOOLS Click the Event Viewer, and then System. Scroll through and double click any of the ERROR lines to get a more detailed view. There will also be a link that you can click to go to Microsoft's web site to clarify the error. It's possible that a Windows service or core process is corrupted. If this doesn't put you on the right track, I suggest checking your system memory with MEMTEST86 - download and burn to a CD and boot to that CD. It's also possible that your Microprocessor is overheating and the computer is not shutting down. Check the amount of dust inside the machine. If the motherboard is caked with it, there is your problem.
Things I do to help maintain the 'health' of my computer include defragging the hard drive on a semi-frequent basis. When was the last time your drive was defragged? Click on My Computer, then right click on the C: Drive icon, then Properties, then do a Disk Cleanup. Follow this with clicking on Tools, then Defrag. This process can take a very long time, so be prepared. Computer use during this process may slow it down also.