Checked the fuses for the ABS system?
If its on as soon as the car is started then I would measure the resistance accross all 4 wheel sensors.
If that doesnt throw up a duff sensor then check the battery voltage & alternator charge output.
This link tells you how to flash dash fault codes out for both the engine ECU & ABS system
http://www.everything2.com/index.pl?node_id=1384650
Lee, June 2007