fan speed monitoring on p4t533-c

I have a problem on a system with the Asus P4T533-C mobo: the system has a
P4 2.2 GHz, 1 GB RAM, Windows XP Pro SP1 with latest patches applied, and
the latest version of BIOS and ASUS tools included the Asus Probe utility.
The CPU fan is correctly connected and working and all the temperature
monitors have always been ok (this workstation has been working with no
problem since it was assembled more than 1 year ago), but I had to disable
the fan speed monitor as it records *negative* speeds (e.g. -19000), of
course under the treshold. One day the CPU temperature has got above
treshold; I don't know if it depends on the CPU fan as this has never
happened again and the temperature is always ok till now, but I'd feel
better if I could monitor the CPU fan speed so that eventually I can replace
it if defective. Does anybody know of a similar problem and how to solve it?

Thanx to all in advance!