Internal DNS Server

I need to install DNS server to test some applications that require domain name for the sever to be interen during installation or setup. It does not matter if it is Linux or Windows 2000, I just need to be able to have internal DNS server that would allow domain name definitions like mail.mycompany. DNS servers wizards usually try to help me to connect my server to the world, but I just need to build “my own world” withing the office network for test puposes.
Anybody can suggest, how can I do that?