If people complain of their system running about HALF speed, it is due to the WMIPRVSE.EXE virus.
Normally, wmipvrse.exe is a valid windows/system32 file, and with SP1, it stays in the WDEM directory at about 199KB, and has the SP1 distribution date of 8/29/2002. But when you get the virus, you will find another file, of 38KB size in the Windows/Prefetch directory, with the same name, but a more recent date. Deleting that file and rebooting seems to fix the problem, the original MS file seems unharmed.
The way I found this is, as soon as the system booted fully, I did C-A-D to bring up Task Manager. A few seconds later, it popped into view, and the system process meter went to 100% immediately.