Ok, you have 2 options. I just tried this on a box in my inventory so I know it works. Get into the bootrom menu and hit d then f. Thats all, d forces the default config and f boots from flash. Now, if you have a box with software versions prior to 4.0 (the 2. series) the only option you have is to configure and connect a modem to the box and have extreme technical support dial in and reset the admin password for you. Oh yeah, you can also send the box into extreme and they can recover it then mail it back to you. If you have a early version box and don't have Extreme support, I bet your screwed and will have to pay them a great deal of cash to have them reset it for you. Hope this isn't bad news.
Brian