I'm trying to get the OneCare team to provide an official announcement about the forthcoming update that is supposed to address many of the upgrade problems.
If OneCare is running, the update will be received automatically.
If OneCare is not running, you'd want to download the latest version and install it after the version is updated to include the fixes.
In your case, since OneCare won't start, you should look at this post - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2523777&SiteID=2
And, since OneCare is not working on your PC, you really should contact support if the above does not resolve the problem as your PC is currently unprotected, which is not good.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve