Since an 1800XP runs at 1533mhz, which equals 11.5x133fsb, and a 2400XP runs at 2000mhz, which equals 15x133fsb, in addition 15x100 equals 1500mhz, which would be recognized as an 1800XP; therefore, I surmise your front side bus is set to 100 instead of 133 as stated above by Stiffler.
All the way back to the Pentium II processor, Intel has put bus speed detection pins on their CPU's. AMD still hasn't learned yet that this really needs to be done. So since no AMD board can detect whether the CPU is a 100MHz, 133MHz, or 166MHz bus version (DDR200, 266, 333), you actually have to TELL the board what bus speed it is. You've already had a couple guys tell you to set the bus speed to 133MHz, and they are right. But just in case you didn't believe them or mistook what they told you to do as overclocking, this explaination tells you exactly why the change is the responsibility of the person installing the chip.
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