try disconnecting the battery for 5 minutes and pressing the brake pedal for
about 10 secs during that time..if that doesnt work then pull all fuses
labeled ecm for 5 minutes if that doesnt do it then go to the zone and see
if they will clear them for you

This is from "Xterrabull" in the xterraownersclub.com message group:

Actually, on 2000 Xterras there is a way to do it via the ECU.
The procedure is as follows:
First, turn the ignition key to the on position.
Then, locate the recessed miniature-phillips head screw on the
passenger side of the ECU (you probably will have to move a bulky,
taped up bunch of wires & punch through or remove a clear plastic cover
on the ECU over the screw).
Once this recessed phillips head screw is located, use a micro phillips
head screwdriver to rotate the screw fully clockwise (it will rotate
approximately 180 degrees or so); then, wait at least 2 seconds, &
rotate it back fully counter-clockwise to the position it was
originally in.
Now, if you have any fault codes, the SES light should blink a series
of slow flashes followed by a series of rapid flashes to give the code
of the fault (non-OBDII standardized codes). If there are no faults,
the SES light blinks 5 long pulses, then 5 rapid pulses.
Now, with the key switch still in the on position, repeat the rotation
of the small phillips head screw as in the above step (once again, the
procedure is as follows: fully clockwise, wait >2 seconds, then fully
counter-clockwise).
Now, turn the key switch off. The next time you start the car, the SES
light should not be on (if it is, then it is still detecting faults).
Also, I believe this may reset the learned behavior of the ECU in terms
of managing air/fuel under load vs. RPM, so there may be slightly
rough-running of the engine for some small period of time (i.e. 15
minutes) when the truck is first driven after reseting the codes (I am
not 100% sure of this, though).