Peterh, I've now answered my own question, having now downloaded and installed iTunes/QuickTime on to my WinXP machine. You can switch between the iTunes player (QT) and WMP via the Program Default settings in Add/Remove Programs.
Just been configuring QT. Is it necessary to register QT (there's a registration tab in QT's Preferences), or do you need to do that only if you're interested in getting info about QT Pro?