If you don't know the administrator password for the PC, I'm not sure that you will be able to resolve this issue without some brute force efforts, assuming that all users of the PC are defined as limited users. You can try the cleanup tool to remove it, but then you may also end up with bigger problems as most security software will require administrative privileges to install.
How to Uninstall Windows Live OneCare
Cleanup tool - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=828910&SiteID=2
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve