This could be due to a problem with the installation, a corrupt .Net Framework, a required service being disabled or not started, or something else broken in OneCare.
If you have rebooted the PC and it still won't open and you are not encountering any errors, I'd consider reinstalling or contact support.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve