Does your computer make any bleeping sounds when it tries to start, as different combinations of bleeps can often diagnose the problem, or doesnt it even get that far.
Normally, your computer should emit a single short beep at startup.
Also, have you cleared the CMOS. (this can easily be done with either a mobo jumper, or by removing the mobo battery for a short while. Refer to your user manual for recommended method)