Rundll is a generic process that is used by other programs.
You would not want to block all instances of rundll32, so the question is what settings are you using for the firewall? Are you asking OneCare to notifiy you for all access or are you only being prompted for unknown programs? If the latter, then there may be a problem that you should contact support for help with as OneCare should not be blocking rundll32. If you are being prompted for any program, then the block and ask is normal and you should probably allow it.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
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