You look in your BIOS and make sure your RAM is set to the appropriate speed? If your CPU is a 400FSB CPU then your RAM needs to be on a 1:1 ratio with the CPU
On most BIOS's with adequate options theres an option that says 'CPU
RAM' inwhich you can choose the ratio 3/3, 4/4, 5/5, 6/6 it's all 1:1 but it offers all those ratios incase you need on slower or faster than the CPU and such.
So go through your BIOS make sure your CPU FSB is where it should be at and the same goes for your RAM. Check to make sure theres a DRAM Frequency option and change it to where it's supposed to be.
Use CPU-Z to find out your current ratio between CPU and RAM