Win 98 is easy. There are many you can use but JR transmitters are a bit fussy. I'd recommend the serial port version as on http://www.customelectronics.co.uk/fmscrktj.htm

Some people have got an even simpler parallel port interface working with JR but I never managed it.

If you want to use it with XP you'll need a more complex version that uses a PIC microcontroller. You've already got the diagrams for that, they come with FMS, look in Doc/Interface.

HTH - Steve