Why is my Athlon XP running below rated speed?
9 times out of 10, when people install a new Athlon XP they find that it runs below the speed it's meant to. I don't mean it runs slower than the Quantispeed rating would imply (though it will, and is supposed to). No, I mean it will really run below the intended stock frequency.
This is because most motherboards like to play it conservative. They set the Front Side Bus to the minimum 100 MHz (200 MHz after double-pumping). And most Athlon XPs are meant to run at a faster speed. Luckily, the fix is simple enough.
First you need to find the intended FSB speed of your particular processor. This information may be provided by whoever sold you the processor; if not, you may wish to consult The Big Processor Guide. If all else fails, find the OPN on the chip and throw it into a Google search.
Once you have this information, you need to change the appropriate setting in your BIOS. Entering the BIOS generally requires hitting some key (such as Delete) as the system boots. Once in the BIOS, you will need to look for a setting labeled "Front Side Bus", "System Bus", or something of that nature. Every BIOS is a little bit different, so the exact steps to get there will vary. Once you locate this setting, you need only change it from 100 to whatever is appropriate, save changes, and exit.
In some cases, the BIOS may not control the Front Side Bus speed. Generally this means there is a jumper on the motherboard that must be changed. Consult your motherboard's documentation.