Domain Controller Not Available

I have an IBM think pad 770 with a dlink ethernet card running Win. 98.
Whenever I try and log on to our network from our location connected with an ADSL line, I get the message (title of this question) and I can only log on locally.
Whenever I try and log on to our network from another location connected to the backbone (standard ethernet) I can log on without a problem.
I have checked the ethernet jacks, IP addresses, and gateways with other machines and there are no problems.
I s there aproblem with this particular ethernet card and the slower (ADSL) connection? A timeout value I can increase?
All thoughts are appreciated.

Thanks,
Stephen