I've noticed that for years. It's something of a mystery why that issue was never addressed by MSFT.
Say, did you have a question to ask?
My XP SP2 system has lately had something stealing CPU time.
It often shows up as "50%" of CPU time used, but the graphs show 100% and that's how the system acts. (even mouse movements are erratic).
But the idle process can show 99% and none of the listed processes are using the time.
I've especially had this problem when trying to use an add-on SATA card, but other times as well.
There's usually no problem when I first start up the system, but after a certain amount of time. I've already tried Norton Virus scan, Windoctor and Diskdoctor.