What happens if the Windows '98 machine is cabled directly to the router - any difference? I have it right at the back of my mind that it matters whether you have First or Second Edition '98 but can't quite remember why. Maybe someone else has an idea.
Any text or figure you puti n there will show as the Gateway but it doesn't mean it's he righ tone. Take the precise Gateway from an ipconfig call in one of the connected machines and duplicate it in the Presario.