There is no way to reset the password without resetting the whole router. Try checking your desktop for, or your computer for a file called: "Network Security Settings". Or check the jacket the router's CD came in. Im going to guess you used the CD in the first place to set up the router, which by default stores the network security key in those two places.
If all else fails, you can reset the router back to factory default by holding the the reset button on the back with a paperclip or something. I would only recommend that as a last result, since you then have to go back in and resecure the whole network.