I suggest running a disk error check. Right click on your drive in Windows Explorer, select Properties, then Tools. Run the disk check for errors. If that doesn't help, I suggest contacting support.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
If the PC is a laptop, make sure that the vents are clear to prevent overheating. Defrag and Scan are both intensive operations and can cause a great deal of heat build-up. A freezing system can be a symptom of excessive heat, disk errors, or conflicting software.
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator