There are many reasons a computer will continue to reboot itself. The usual reason is because when it reboots, it is attempting to repair itself. If it should continue to reboot you need to find out why. Below are some troubleshooting steps that should aid in your hunt to find the culprit:
1. Open the computer and remove all dust from the case. Often dust will create insulation and cause the computer to overheat. Overheating is a definite rebooting suspect.
2. Insure that all of the case fans are operating. You can do this just by sight and sound. Again, if one or more fans are not operating, this will cause overheating.
3. Test your RAM (Random Access Memory). There are many places where you can check to see if the memory you have is operating correctly. Here are a couple to choose from:
http://hcidesign.com/memtest/
http://www.memtest86.com/
4. Your power supply could be going bad. It can be checked by using the following procedure:
ATX power supplies have a simple diagnostic circuit that you can use to determine if your power supply is good or bad. Here's how.
Tools needed: Voltmeter.
Difficulty: Average
Time Required: 5 minutes
1. Shut down your computer and open it up. Leave the power supply connected to the AC power cord.
2. Leave the power supply's master power switch on, if it has one.
3. Disconnect the ATX power connector from the motherboard. This is a wide, flat connector with two rows of pins and a locking tab.
4. Locate the pin connected to the gray wire. This is the PWR_OK pin.
5. Locate any pin connected to a black wire. These are the ground/earth pins.
6. Place the red (positive) probe of your voltmeter on the PWR_OK pin, and the black (negative) probe on any ground pin.
7. If the gray pin reads 2 volts or more, then the power supply passed its internal diagnostic. Your power supply is probably good.
8. If the gray pin reads much less than 1 volt, then the power supply is dead. Replace the power supply.
Tips: If the power supply tests OK, then your PC problem probably lies elsewhere.
Motherboards check the PWR_OK pin before they start up. If a power supply fails the test, then it cannot be used in an ATX computer system.
Some power supplies might not activate the PWR_OK pin properly unless they are under a slight power load. If the PWR_OK pin shows little voltage, try shutting down the PS and plugging a peripheral (CD-ROM, floppy, etc) into the power supply, then turn it back on and check the pin. If the PWR_OK pin shows voltage, then the PS is probably fine.
5. Test your Motherboard. Many Motherboard manufactures have their own testing software, so try them first.
6. Checking hardware with known good hardware is a common troubleshooting method.
Note: Testing hardware can be very involved and can be very risky. Opening up your computer system is not for the beginner computer user. Be sure that you take every precaution when attempting any task involving opening your computer case.