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Provided your version of XP is up do date with all the service packs, up to service pack 3, then you should have no problem connecting to encryped networks, unless there is an issue with the wireless card drivers, or the wireless key is wrong.
You need to make sure that any old drivers and wireless connections from the old card are fully uninstalled, to avoid any conflicts. Also make sure that Windows is set to manage the wireless connections, and not any 3rd party wireless utility.
I took your advice and used Windows Zero Configuration (WZC) to configure the new wireless n card. I had been using the software supplied with the card (from RaLink).
My XP PC is now connecting to the Internet through the Hub5.
Thanks for your help.