Joiniig Vista Business to Windows 2000 Server Domain

at work we're still using a Windows 2000 server as our DC. We have rolled
out our first Vista laptop, and we're having trouble getting it to join the
domain.

When I try adding the computer to the domain I get the following error: "The
format for the specified network name is invalid".

I am typing in the domain name correctly, and the computer name is simply
the user's last name.

Any help would be gratefully appreciated. Thanks in advance.