Networking WinXP to a Win2000

I am unable to access my Win2000 computer from my new WinXP (medie center edition). I have both computers on the same workgroup. I can see my Win2000 computer from my WinXP computer, but when I try to access it I am prompted for a username/password, which I enter as I would on the Win2000 machine, and then receive the following error msg,

“xyz computer is not accessible. You might not have permissions to use
this network resource. Contact the admin of this server to find out if you
have access permissions. Logon failure: the user has not been granted the
requested logon type at this computer”

I can access my WinXP machine from my Win2000 machine.

Any thoughts?

Thanks,
Chris