screen going black

Try inserting a Boot Disc into the Optical Drive and see if the monitor display stays on.

If it does the problem is that the Installed OS has been corrupted and needs reinstalling.

If it doesn't you have a hardware problem that needs investigating.

For a Boot Disc if you don't have one you can use a Live Linux which you can either download from here

http://www.livecdlist.com/

Or you can walk into your local Newsagent and buy a Linux Mag with a Live Linus on the Cover Disc.

Or you can use the Ultimate Boot CD available for free download from here

http://www.ultimatebootcd.com/download.html

Col

Try changing the Power Supply it most likely is going off and not producing the correct Voltages or Current.

I personally use Antec Power Supplies as they are rated to a Constant Load so if one is rated as 350W that means that it can supply 350W 100% of the time without any problems. The cheaper No Name PS's are rated to a Peek Value just like some Sound Systems and that means here that a 500W PS can produce 500W of power for short periods of time and then has to drop the output Current and cool down before it can produce it's Rated Power again.
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So in a case like that a 350W Continuous Power Supply will be equivalent to something like a 500W Peek PS. ;)

Col