Default Gateway on Windows 98

I have a ADSL Modem (IP.192.100.9.200) & a ISDN modem (IP 192.100.9.2), both connected to the D-LINK switch. A Windows 2000 server is configured as the DNS & DHCP server, my default gateway & DNS search is configured to the ADSL modem.

However, few Win98 PC always connected to the IDSN modem, eventhough I manually configured the station with a static IP add , and define the gateway to be of ADSL.

Else, those WinXP, Win2K station are fine.
Pls help.