repeatedly prompting for username password mapped drive

I am using an as400 to share some files and folders across my
workgroup. All workstations are XP Pro. For the most part it works
well but I have two computers that do not work at all.
What I have done is mapped a drive to the shared folder and specify to
log in using different username/password. This has worked on most PCs
so far.
However, I have run into 2 machines that continually prompts for the
username/password over and over. It is like the credentials aren't
being sent or are being sent incorrectly and therefore the server
rejects the attempt and the login box reappears.
I would appreciate any explanation, fix, or ideas to help solve the
problem.
Thanks,