Without knowing the error you are encountering when trying to install OneCare I can only refer you to general install troubleshooting.
See this post for the KB article to troubleshoot install problems - http://social.microsoft.com/forums/en-US/onecareinstallandactivate/thread/ba26b781-da18-4ce4-be8c-22647bd5d8fc/
http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
If you are using Windows Live OneCare and you have been infected, but OneCare did not detect or cannot remove the malware, please contact support to report this and for help with removal.
How to reach support (FAQ) -
~ Microsoft MVP Windows Live ~ Windows Live OneCare| Live Mesh|MS Security Essentials Forums Moderator ~