What version of JBoss are you using? How did you install it?
I assume that you installed version 4.0.3 (or 4.0.3SP1) using the installer. As part of the installation process, you were presented with a form to set the security for the app server. On that page, you were asked for a user name and a password. What did you enter into those fields? If you don't remeber, the easiest thing to do is to install JBoss again and this time either don't set security of remember what you entered.
The other way of installing JBoss (and the only way of installing JBoss before 4.0.3) is to unzip or untar/gzip the software into a directory, and in that case there is no security applied by default. So if you installed JBoss in this fashion, you must have also changed the security setting in the configuration files.