IIRC this only works on facelift models 99 on so should be OK on your 2000, I know it doesn't work on my 97 A4 which has to be done via VAGCOM:
1. Insert your key into the ignition.
2. Push in the odometer reset button, hold it in, and turn the key to the first click.
3. The display should read either "oil" or "service" if it does not, repeat steps #1 and #2.
4. To reset, pull out your instrument light dimmer button at the bottom of the tachometer.
5. Turn the ignition to the off position. When the ignition is turned back on, the display should be back to the trip odometer.