You were right in thinking that having McAfee and OneCare on the PC at the same time can cause a conflict. Now, I'd make sure that there are no remnants of the McAfee protection in place.
McAfee Removal Tool - http://www.majorgeeks.com/McAfee_Consumer_Product_Removal_Tool_d5420.html
Also, be sure that no other active security is installed and running.
If the problem persists, please contact support -
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve