XP and ISDN router

Hi

I have a numbers of Windows XP Home systems which are
part of a small office network which uses NetBIOS for
file sharing, also part of the network is a router which
allows us dial on demand ISDN line access to the
Internet.
My problem is that every time the XP workstations are
rebooted they initiate a DNS lookup for the computer name
using remote DNS servers which causes the ISDN to connect
and access the Internet. As the ISDN line is expensive we
are trying to limit access to just required Internet
access so doing a worthless DNS lookup everytime a
workstation is booted is not a good idea.
I assume the DNS lookup is part of the NETBIOS mechanism
and have configured the LMHOSTS file to include the Local
computers IP address and computer name with the PRE flag
set.
My question is why are the computers still doing DNS
lookups at boot time even if the data its looking for is
already defined in the LMHOSTS file ?

Thanks for any help.

/Michael