Windows 2000 full cpu load issue
Compaq Evo D510 SFF with Microsoft Windows 2000 was not working, investigation showed cpu maxed out - all in the System process. Process Explorer showed 100% cpu split between System and DPCs, which seemed to indicate a device driver problem. A bit of internet searching led me to try disabling the USB drivers, but in the end I only had to disable "Intel PCI to USB Enhanced Host Controller" in the Device Manager to fix the problem, leaving USB still working.
Initial help
MS Knowledge base (similar issue)
More help