When you installed OneCare, you may have entered a Windows Live ID, but not activated it.
When you try to activate your OneCare subscription using a different Windows Live ID, the Windows Live ID may be greyed-out and you are unable to update or change it.
Run the following tool to reset your Windows Live ID in a trial copy of OneCare:
http://download.microsoft.com/download/f/7/5/f759f03c-cee7-4e3d-978c-0d56b9a8533b/RemoveMachine.exe
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Click Save.
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Select a location where you want to save the file, and then click Save.
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Go to the location where you saved the file.
In Windows XP, double-click the file.
–or–
In Windows Vista, right-click the file, and then click Run as administrator.
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Click Run.
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If you're using Windows Vista, click Continue.
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When the repair tool starts, click Continue.
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Read the licensing agreement. If you agree to it, click I accept the terms of the End–User Licensing Agreement, and then click Continue.
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Click Start.
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Follow the on-screen instructions.
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When the tool is complete, open OneCare, and then click Activate.
If the above tool fails or you encounter any problems, please contact OneCare support.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve