Could be alot of things
1) Sometimes this can be caused by an overheating situation. Itunes, RealPlayer, Windows Media player, etc. require a lot of processing power to decompress and decode music files, which can cause the processor or hard drive to get hot.
If your CPU is running at over 60 degrees (C) you might be at risk of burning it out. Some systems shut down automatically when the temperature reaches an unsafe level. Every few weeks I open my system unit and clean the fins on the heat sink that sits under the CPU. When they collect dust it restricts the airflow and prevents proper cooling. You can use a can of compressed air (look at your local office supply store) or an old toothbrush.
2) If you determine that overheating is not the problem, the most likely suspect is bad memory. Trying to access a bad spot in your system memory (RAM) can cause the computer to freak out and restart. The best way to find the culprit is to pop open the system unit, remove (or replace) one RAM stick and see if the problem is solved. Run your system for a while and if the problem goes away, you win! If not... lather, rinse and repeat for each RAM stick until you find the one that's misbehaving.
3) There's a setting buried in Windows XP that tells your computer to restart when a system error occurs. If you turn off that option, you may solve your automatic reboot problem.
# Click Start, then open Control Panel
# Click Performance and Maintenance
# Click System
# Click on the Advanced Tab
# Click Settings in the Startup and Recovery section
# Uncheck Automatically Restart in the System failure section
Note that this may prevent the system from restarting, but it can also mask the true problem. As an alternative to this measure, consider what has recently changed on your system. If you have installed new hardware or software, remove it and see if the annoying restart persists. Sometimes downloading the latest driver software from the manufacturer's website will fix hardware incompatibility problems that cause restarts.
4) Of course an obvious answer would be spyware/virus/trojan that is causing it to do so. I once heard that the Sasser worm caused a computer to go into a loop of restarting. Look into Avast, Zonealarm and Ad-Aware to help with those issues.
***Answers 1 2 and 3 were gathered from other sources***
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