Win2k Servers & Win9x Clients

Hi,
I’ve just finished upgrading my companies SBS4.5 Single Server to a Multi Win2k Server system.
We have a mixture of client OS’s, Win2k Professional, WinNT4, Win98 & Win95.
The Win2k & WinNT workstations seem to work without any issues.
However the Win9x machines daily seem to get themselves booted off the network.
User will attempt to access ‘Network Neighbourhood’ and will be told that domain xxxxxx.com is not accesable.
They are then required to re-boot.
Any ideas?