By far the simplest solution is to plug the old hard disk into the new system, change the jumpers to make it a slave and then access the data you need. Next level up would be to use an Ethernet cross-over cable between the two PCs and map a network drive from the XP to the 98 PC. You'll need to make sure all the security stuff is switched off but that won't matter as long as you aren't connected to the net.

There are paid for solutions such as Laplink but you don't need it for what you are doing.

You might need to extra advice dependent on the type of data you want to transfer, but we'll deal with that when you are ready.