Vista CPU Frequency not shown correctly, even when SpeedStep is working OK

I have a problem with my Vista Home Premium installation where the CPU
Frequency always shows the maximum available speed (1.66 GHz for my
T5500 Core 2 Duo processor) and voltages. On the Vista Resource
Monitor, the Max Frequency (blue line) is shown as 0%. The
Performance Monitor tool shows it as maximum. The WMI raw performance
figures shows it as maximum and never changes.

If I use a tool that shows the actual CPU speed/voltage by looking the
processor itself, it appears that the CPU is being correctly adjusted
when the CPU goes idle, so it EIST appears to be enabled OK.

I've just done a clean re-install to see if this fixes it, but still
the same.

Anyone seen a similar problem? Does anyone know which bit of Vista
supplies this performace data (WMIPerfInst?) and how to diagnose why
it's failing?

Thanks,
Neil.