Greetings --
There's no requirement to register WinXP at all, so I'm guessing
that you're really asking about activation.
There's no limit to the number of times you can reinstall and
activate the same WinXP license on the same PC. Nor is there ever a
charge. If it's been more than 120 days since you last activated
that specific Product Key, you'll most likely be able to activate via
the Internet without problem. If it's been less, you might have to
make a 5 minute phone call.
Here are the facts pertaining to activation:
Piracy Basics - Microsoft Product Activation
http://www.microsoft.com/piracy/basics/activation/
Windows Product Activation (WPA)
http://www.aumha.org/a/wpa.htm
Bruce Chambers
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