W2K Pro joining W2K Server Domain

This has been a hit and miss for me for several months, everytime I try to join a computer to my domain, I will successfully make it in or get an error:
The following error occured attempting to join the domain “XXX”: The credentials supplied conflict with an existing set of credentials.

I use the Admin username & pw, hoping to be by this however no luck, and haven’t been able to pinpoint the root cause of this. Any help is desparately needed..