A new computer uses the MSI 845GE Max motherboard with a Celeron 2.0GHz CPU. Since the Celeron 2.0GHz can easily overclock its external clock to 133MHz, multiplied by the CPU's 20x multiplier, it becomes 2.66GHz. Thus, in the motherboard BIOS, the external clock was adjusted from 100MHz to 133MHz. After saving and restarting the computer, the POST passed successfully, and the system entered Windows normally and ran properly. When using WCPUID for detection, it was found that the CPU was indeed running at 133MHz, with a main frequency of 2.66GHz. However, after some time, the computer became extremely slow, and restarting did not help; the CPU temperature was also normal. Later, upon re-entering the BIOS and changing the external clock back from 133MHz to 100MHz, the system remained extremely slow. With no other options, the hard drive was repartitioned and the system reinstalled, along with MSI's built-in automatic overclocking software, FuzzyLogic 4. When the CPU was set to 133MHz again, the system once more became extremely slow. Why can't this Celeron 2.0GHz CPU be overclocked? And why does it still not work even when the external clock is restored to 100MHz?