Win2000 to recognise my server domain

I’ve just purchased a DELL workstation with Win2000 as the operating system. I tried to put in my server domain name by right clicking on my computer, select Properties and select domain instead of workgroup. Then I type the domain name NTDOMAIN_12 and it failed. Does it mean that the domain name can’t have a underscore? Comparing to WinNT, it is quite difficult to configure it to work on the lan. Would appreciate if someone could assist me. Many thanks