I think you should reinstall the operating system first~if it still have problems,then try other kinds of OS,such as Win98,Win2k,Linux,BSD or something else.
IF it still have problem ,it must be your hareware's problem.Try other one mainboard,memory,and power supply,one by one~DO NOT change it all at one time.~
If you still cannot find out what's going on~then call your local company ,friends whatever ,solve it.It will more easy way to have troubleshooting when somebody can see the message on the screen.