Gateway Desktop Model 0021431401 Wont Boot up

The power turns on and the monitor lights up to the traditional “gateway screen” where it says push the tab button to enter system setup. Then it freezes, however you can open and close the CD drive doors and you can shut the computer off by holding the power button – but thats it. I tried putting a floppy disk into the 3.5 drive to see if it would pick it up as a default boot method but nothing happened. I tried putting in a new operating software “windows xp” but that didn’t pick up either. I need some ideas.. the key doesn’t respond either – other than it lighting up the 3 green lights when you first turn the computer on. HELP .. I got a bad feeling that it could be the CPU?