Shuts down as in turns off? Does it go through the windows shutdown procedure or does it just lose power?
Also, what Op System are you running? Is there anything that you're doing when it cuts out? Does it give you any error messageS? What power supply is in it? Is this a pre-built system? Dell perhaps? More information please. How old is it? I gather is fairly old.
It loses power without getting any errors or anything else...
Iam runnig Windows 8 Release preview Build 8400.....
Sorry i dont know what power supply is it Can you tell me how to know
I bought my pc a 5 months ago
How long have you been running Windows 8? Perhaps your computer isn't up to snuff to run it.. Also if its a preview build, its not 100% stable.
Finding out what power supply you have involves opening the case and looking at it, I'm going to guess its a cheap crappy one. Can't guarantee that is the problem though. The best way is to swap in a known good power supply into the system and see if the problem continues. How often does it lose power? Have you tried another outlet in your home? I know that sounds silly, but you'd be surprised how often that happens.
I'd still suggest installing a stable operating system, sounds like an awful old machine to be running Win8.