Kerberos authentication error

Hey all, I'm trying to join my 12.3 computer to my 2003 domain at work. After a long Linux dry spell I booted my workstation and received an error message stating that my Kerberos ticket has expired. I clicked Get Ticket and after about 20 seconds received the following error message:

I guess my main question is how did I do this in the first place if the configuration file doesn't have the "default realm" specified? I've been Googling this error without any success and I couldn't find anything on this forum that applies. I have a basic knowledge of Linux but this project put me out in unfamiliar territory. Any ideas on what the problem is and how I can fix it?

Thanks,

Joe B