A little more specific information about the problem would help troubleshoot what is going on.
 
Does the hang and reboot happen when the system is booting into the OS, or when you are in the OS and performing certain tasks (games, surfing the net....)?
 
What exact OS are you running? Windows XP, Vista...?
 
Is this a system that has been running fine for a while, or a new build?
 
Any new hardware or software that has been installed recently?

First thing first thanks for reply .I'm using windows xp and my systems when i'm working at my system like surfing,gaming or working any application .

Hi,
i think you shuld check the event viezer for the reason of rebooting.
Right click on my computer , select manage, and see the event which caused the system to reboot.
 

Do you hear any strange clicking coming from inside the machine before it reboots? I had a similar problem to yours where everything worked fine and then this clicking started, which froze my machine and forced a reboot. This got progressively worse until I couldn't even boot into the OS.
 
If this is your problem, then your HDD has a physical error on the disc itself, meaning your HDD is dying. This is not repairable, so you'll have to get a new HDD and transfer the files.
 
NOTE: If you are not experiencing these clicking noises, then this does not apply to you. I'd hate to have you read this and assume you need a new HDD, which can be a little expensive.