Following successful reinstallation of W98SE (finally) I now have the problem of my external modem - a Multitech Systems MT5600ZDXe. I have installed the driver but it is looking for a 5656.cat file - where do I obtain one of those?

try using the search feature in windows. It's been a long time since i used Win98 but I believe you'll need to check an option for hidden files and folders in the search.