Recently installed one care and now can't run defrag using oncare or microsoft xp defrag utility.
Perhaps One Care did not install correctly. I suggest uninstalling One Care, running the install cleanup tool - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=828910&SiteID=2 then reinstalling One Care.
If the issue persists after reinstalling please contact support. How to reach support - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2