Based on your description, you have downloaded and installed updates from Windows Automatic Updates. However, you may also have updated OneCare at the same time.
If the PC won't boot due to the update process being stuck, see here for support options for Windows Update:
Windows Update Solution Center - http://support.microsoft.com/ph/6527
And you can also contact OneCare support:
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