ORAC,
Thanks for the G7toWin link - a handy little program.
My problem turned out to be simple, once I'd found what it was!
I have Microsoft ActiveSync running to synchronise my PDA with my PC, and it was that which was messing up the serial port comms. Once I killed its process (wcescomm.exe) everything was fine. I only have it installed under W2K, not W98, hence the difference.