Many Dell Inspirons use a P4M processor that can adapt its speed depending on the cpu load and the remaining power of the battery. Like Many other Dell Inspiron users, I recently had my 2.4GHz processor totally stuck to 1.2GHz and it was not only a problem of speed reporting since my builds took SIGNIFICANTLY longer than usual... I first tried to update SpeedSwitchXP to its latest version (if you don't use that on a laptop with a mobile proc and WinXP sp2, you're missing something...) and it did not help. I tried various utilities like cpu-z and Rightmark CPU Clock to check or tweak the real cpu speed. No effect.

Eventually, I rebooted into BIOS setup and disabled SpeedStep, saved options and rebooted. Then rebooted again into BIOS setup, re-enabled back SpeedStep, saved and rebooted. And it fixed the problem. Don't ask me why or how, but this disable/enable dance helped a lot and my proc is back to its full 2392.2MHz normal speed on AC instead of acting like a snail.

Hope that helps if you have the same issue with your laptop...