Computer freezes or reboots at random intervals.

Possible fixes:

Update Windows. Ensure you have the latest service packs installed.

Update all Device drivers. Driver issues, especially for video drivers, are a common cause of system crashes.
Scan for viruses, malware, spyware. Click here for a complete list of free utilities that will perform a thorough scan of your system.

Re-seat all cards & plugs.

Test your RAM. Use this utility, and let it run overnight for best results.

Test your HDD. Click Start --> My Computer. Right-click the disk you want to check (usually C: ). Select Properties. Click on the Tools tab. Under Error Checking, click Check Now. Select the Automatically Fix File System Errors option, and the Scan For and Attempt Recovery of Bad Sectors option. Click on Start.

Defragment your hard drive(s). Click Start --> All Programs --> Accessories --> System Tools --> Disk Defragmenter.

Confirm that system temperatures are not reaching critical levels. Processors are especially susceptible to overheating, but other components can also cause problems.Consult the forums for diagnostic tools that will analyze your system temperatures.

Ensure your power supply is adequate to the needs of your system. If you have purchased a bare bones system to save a few bucks, and have added components over time (drives, video cards, etc.), these add-ons may be making demands that your power supply is unable to handle. Determine the wattage rating on the power supply in your machine; the label on it will usually provide this information. Then click on this link to calculate your system's requirements.

Try to recall any changes you have made to your system. New hardware or software will sometimes cause conflicts, creating problems where none previously existed. If you begin having issues and can pinpoint their occurrences to a time immediately after a change was made to your system, the new hardware/software may be the problem.

Clean the registry. A Google search will direct you to several free utilities that will scan and clean your registry.