If your backup does not have a progress indicator and a cancel button or you cannot cancel the backup by clicking cancel, reboot the PC to get it to stop.
If you encounter any issues with this, please contact support.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
Microsoft MVP Windows Live / Windows Live OneCare & Live Mesh Forum Moderator