Only the Mobile Intel Pentium 4 M processors have SpeedStep technology. As I know it, the Dell Inspiron 8500 has a Mobile Intel Pentium 4 M 2.4 Ghz processor.
What does the BIOS identify the processor as? (As in the name / model).
The best thing to do to is boot into Windows and run CPU-Z
, to make sure the the system has identified the processor correctly. You could also try running some power hungry applications and see if the CPU speed throttles up.
Also, check to see if there's a CPU throttling program running (one that lets you choose different performance profiles like power saver and high performance, thereby restricting clock speeds).