This little app works a treat.
From the download site:
"CPU Speed is a small, standalone tool that measures the actual speed
of your Intel or AMD processor (CPU). It requires no installation,
just run the executable and start the speed test. The determined
speed may not reflect exact speed as indicated by your system
information (since other processes may take up CPU resources), but it
should be fairly close to it"
Runs on: 98/ME/2000/XP
Get it here.