As they are both connecting through the same router they are on the same network, easiest way is to share out a folder on one and copy it down to the other
On windows xp machine > open windows explorer. (Right click on start button and select explore)
In windows explorer browse to my network places. Right click on my network places and select explore.Browse to entire network. then microsoft windows network and then to workgroup.
Alternatively find the ip address of the windows 98 machine. Use to find the ip address of the w98 machine. On the windows xp machine. open up a command prompt. (Start > Run > Type cmd in text box > then click ok.) When the command prompt opens type ping xx.xx.xx.xx where xx.xx.xx.xx referes to the ip address of the w98 machine.
so if your windows 98 machine has an ip address of 12.13.14.15
Type ping 12.13.14.15 and then return.
If the request times out you cannot 'see' the other machine.
This site will give your public ip address which will be the outside address applied to your router, if you have both machines connected to the router they will be on private addresses and using DHCP eg 10.x.x.x or 192.168.x.x
On win98 machine go to start, run and type in winipcfg - that should pop a windows ip configuration screen
On XP go to stsart, run and type cmd - this will open a dos window
In the dos window type ipconfig /all - this will give the ip of your xp machine