This could be caused by conflicting software such as other security software or previously installed security software that did not uninstall completely. Uninstalling other security software, if present, should resolve this isue. If you have uninstalled security software prior to installing One Care check the removed software vendor website for an uninstall tool. Support can also help with this issue. How to reach support -
http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
Jim - MVP Windows Live - Forum Moderator - Live One Care - Live Mesh