Win 2K Svr login domain name question

I’m a student of Win 2K & 2K3 server, and this my be a rediculous question.
After a recent move from an Win NT4 server to a Win 2K server(AD), My Win NT & Win 98 PCs wouldn’t correctly login to the same domain name(Domain.com)format(without a failure to find domain error, but they were visible, but file transfer was iffy). The Win 2K & XP Pro PCs logged in perfectly. I have noticed that Win 2K server is really slow & sluggish. Browsing the network from the server could take a minute to display computers. To get the NT4 & Win98 PCs logged in, I finally found that I had to use the Domain alone (no .Com). I tried a Win 2K WS on Domain only & it seems faster too.

My question is, should all workstations in the domain login using Domain or Domain.com?
Thanks
Ed