If you do not see an option on right click for "scan for viruses" when right clicking a file in Windows Explorer, but OneCare works in all other ways, it would seem that the shell extension is broken. You may want to try reinstalling OneCare to see if that helps. Before removing OneCare, check to insure that you know the LiveID for your subscription - open OneCare and click Help/About... The subscription ID will be needed to re-subscribe after the reinstall.
How to Uninstall Windows Live OneCareInstall the trial from
http://onecare.live.com and subscribe with your LiveID.
If that does not resolve the missing right click option, please contact support for help.
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, & MS Security Essentials Forums Moderator