do you run jboss together with oracle on one machine ?
if you do a full blown installation of oracle ( i know at least for the 9.1.x version ) some service called xdb is installed additionally, which is running on port 8080.
so this user/password request does not not come from jboss but from oracle.
solution: either disable the xdb stuff in oracle or change the jboss port for the web container.
to change the port in jboss, look for the web container config file e.g. in server/default/deploy and change the port from 8080 to 9080 or whatever port you prefer.
for tomcat the name of this config file is tomcat4-service.xml
for jetty i don't know, maybe someone else can post