We found out, that the "Service Temporarily Unavailable" Error is a Problem with the Apache load balancer and not with the jboss server.
But we still have the performance problem with the jboss on aix.
With the information you provided noone will be able to give you a solution. But we can start to ask a few questions. :-)
Is the hardware of the AIX machine and the Windows XP machine identical? If not, what is the difference? CPU speed/RAM/#cores?