It may be due to an infection on the PC that OneCare has missed or it may simply be corruption of Windows due to some other issue, but I suggest starting by contacting Onecare support.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
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