PC will not boot up on fitting new cpu

I have installed a Celeron 1ghz processor into a Asus TUA 266 board I am running windows XP. hen booting up it just keeps restarting. I have up graded the Bios. I have also installed service pack1. If I set the speed to manual and run the CPu at a slower speed and the bus speed below 100 mhz it will boot up, but system will hang after about 20 minutes of use. Could any one please help with this problem it is driving me insane. I have 160 mbit of memory. Had no problem with my other cpu which was a celeron 600 running at 60mhz. Cheers for any help Peter