Thanks for the clarification.
If you haven't already rebooted the PC, please try that first.
If you installed some new software in the last day or since the last reboot, try removing that software or update to see if you can reboot and get OneCare to start properly.
If you encountered this when OneCare completed updating itself to v2.5, you may want to consider reinstalling OneCare. Remove it first from Control Panel/ Add or Remove Programs (Programs and Features in Vista) and then install from http://onecare.live.com and subscribe/activate using your subscription LiveID.
Or contact support for help:
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve