Open the box and look at the CPU
Overclocked CPU? I have seen a few bargain basement PC's that are sold as a certain speed but are really just overclocked.
It could also be a 600MHZ CPU that should be clocked at 6X100 but is incoreectly clocked at 4.5X133. (I believe this was mentioned above).
Your best bet at finding out what the problem is would be to physically inspect the CPU and determine what type it is and what the correct clock speed is.
Intel CPU's have markings on top something like 600/512/100 which indicates clock speed/cache/FSB.
Good luck...