Hi just had the fault with my 1999 Fiesta (Mk 4)
The most common fault is due to the ABS sensor on the drivers side front failing, its easy to check, unplug the 2 pin connector and test with an ohm meter you should have a circuit i think about 1K but dont worry, just check you have a circuit. mine was open circuit, replaced with one off Ebay for �35 job done! Oh one more thing, this threw me for a bit, once you have done it the ABS light will still be on, so drive your car above 30 mph and it should reset!
A bit more technical info if you still have problems, obviously check all the other 3 sensors for the same fault.
switch your test meter to mV and connect to the sensor and spin the wheel you should see a slight reading, one other check, turn on the ignition and check the incoming supply to the sensor, this should be about 2.5v
Happy motoring