This USB adapter looks to windows like a 4-axis gamepad, most likely because that's what it needs to support the PS2 gamepad's 2 analog sticks.
This would be fine, except that the n64 pad's SINGLE stick seems to affect the x, y, z, & z-rotational axis. In other words, if you're looking at the winXP gamepad test screen:
- moving the analog stick a little bit upward will move the crosshairs slightly, but also send one of the z values to FULL or ZERO.