If your computer always restart, There are some reason:
I. It's Hot in There
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.
Memory Fails Me...
If you determine that overheating is not the problem, the most likely suspect is bad memory.
II. Don't Do Me Any Favors
There's a setting buried in Windows XP that tells your computer to restart when a system error occurs. you turn off that option, you may solve your automatic reboot problem.
1) Click Start, then open Control Panel
2) Click Performance and Maintenance
3) Click System
4) Click on the Advanced Tab
5) Click Settings in the Startup and Recovery section
6) 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.
The most common cause of computer restarting is a corrupt registry.
Most likely your registry is bloated and overstuffed with an assortment of useless pieces of data. The operating system while running your computer has to pick its way through all this excess material. There will be bits of programs which were not completely uninstalled, parts of programs which were never completely installed, fragments of miscellaneous data, useless drivers, and a multitude of other bits and pieces.
The answer is to clean your registry and remove all the material which is not needed.
Get the best one, and completely clean your windows registry with a few clicks of the mouse.
III. Computer Restarts After Download?
This can also be a virus or spyware problem. I suggest you go through your Control Panel / Add or Remove Programs list and remove any programs you don't need, then run thorough anti-virus and anti-spyware scans. See: