In Windows 98's Network properties, there is a "File and Printer Sharing" button below the window that shows the adapters, protocols etc. Have you ticked file sharing and, if appropriate, the printer sharing in that box?
Can the XP PCs ping the IP address of the 98SE PC?
Has any firewall software been configured to allow access to the 98SE PC's IP address?
Have you actually "shared" at least one folder in the Win 98SE PC?