There are many, many threads in the forum for this issue.
The problem is typically caused by a conflict with other active security software on the PC fighting with the malware protection engine used by OneCare (and Defender) for CPU time, causing a deadlock. There have been cases when it is not caused by a conflict with security software, but that was some time back and conflicts with chipset drivers, etc., should have been resolved by the antimalware team.
If you are not running any other real time security software, and cannot identify the possible conflict, please contact OneCare support to have the issue investigated.
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 Forum Moderator