Assuming you are looking at the board and the PCI slots are the top, see the upper-right mounting hole? See that little split-circular thing just below it? That's the hardware reset jumper.
1. Take a decently large phillips screwdriver and rest it in the center of the split-circular thing.
2. Power on the router, leaving the screw driver in place.
3. Wait for the double-beep, then try to log in (once you hear the double-beep, you can remove the screwdriver jumper)
You won't retain the configuration, but you can now log in using the default IP/username.