On your WIN 98 computer.....create the share, allow 'all'. LOGOFF the computer and Log back on. (hitting the 'cancel' or 'esc' cancels MS networking on a Win98 computer)
Also, ensure BOTH computers have 'obtain IP address automatically' set. Make sure you haven't inadvertantly put in a subnet, or default router, or DNS server somewhere.

I doubt if it has anything to do with the router....since both PC's get IP addresses.