You need to obtain a file called no_pswd.exe from Nortel Support.
As far as the procedure goes, create a recovery flash with the files ti.cfg and asn.exe (no_pswd.exe renamed to asn.exe) Please note that these are the only 2 files that needs to be on the flash, which will be used to remove the asn's password.
Once the router has been powered up and gone through its post, you'll notice the run and boot led's flashing intermitantly. Waite about 4 secs and then switch the router off and replace the recovery flash with a flash containing the normal asn image and config file.
That's it, your router should now have no password.