If you mean that you cannot "see" the Win98 machines, as in you can't see them in My Network Places, ensure that the Computer Browser service is running on the Windows XP machine. Also you can try opening the share by typing in "explorer \\192.168.x.x\sharename" (where 192.168.x.x denotes the IP address of the Win98 machine) from the command prompt or type "net use * \\192.168.x.x\sharename" to map a drive. Also you can try to do a Find-->Computer from the Windows search feature.
The Computer Browser service obtains a list of computers on your local network through NetBIOS broadcasts. If you open a share using the IP address, TCP/IP is used. TCP/IP is more reliable than NetBIOS since it uses addressed packets rather than broadcasts. On a single network (non segmented) NetBIOS broadcasts should be enough, but try using the IP instead.