When do you get that error? Are you starting OneCare from the Start Menu or doing a right click, scan for viruses in Explorer?
Did something change just before OneCare disappeared from the System Tray? It sounds like something may be conflicting with it and preventing it from running, perhaps another security program.
I'm not familiar with that error and since OneCare does not appear in your System Tray, I think it may be best to run the cleanup tool -
http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=828910&SiteID=2
then reinstall OneCare from http://onecare.live.com and activate by clicking subscribe and using your subscription LiveID.
If you still can't get it to appear or work correctly, please contact support.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve