How can I change the CPU frequency of a Dell Latitude C800 in linux?

"Edit: I see you're on Xubuntu. The scaling applet in XFCE4 is in the ""xfce4-cpufreq-plugin"" package; make sure it's installed.

Looks like that computer uses a PIII(-M?) 1GHz? That should be supported by one of the speedstep drivers in this list:

The Linux kernel CPUfreq subsystem can
only work on a system if the hardware
supports CPU frequency scaling and a
device driver exists.

Intel SpeedStep using the SMI BIOS interface (speedstep-smi)
Intel SpeedStep on ICH-based chipsets (speedstep-ich)
Intel Enhanced SpeedStep (acpi-cpufreq and speedstep-centrino)

Here's a tutorial covering CPU frequency scaling in Linux.

There's a bug open on Ubuntu Jaunty that complains about CPU scaling not functioning on a similar CPU to yours. The upshot is that the kernel may be deciding that frequency scaling may not be useful for your processor. If you're running into this bug, disabling frequency scaling entirely may help you get it running faster.

If nothing else has worked yet, we'll need more information. First let's try to make sure your kernel is using the right driver for your CPU.

Please update your question with the output from these commands:

$ dmesg | grep cpufreq

$ cpufreq-info --driver
$ cpufreq-info --hwlimits
$ cpufreq-info -m -freq

If you don't have the cpufrequtils package installed, install it (""sudo apt-get install cpufrequtils"")."