diskeeper vs Norton ,speed disk ?

A stripped down version of Diskkeeper ships with Win2K/WinXP. When I downloaded and installed a trial of the full version, my system began doing strange things. The hard disk would seem to defrag even when I was busy working on it. Every time this occurred, there was a large CPU hit. I tweaked every setting, but could never get Diskkeeper to be gentle. Norton, on the other hand, obeys its priority settings, and gets out of my way when I resume use of my system.