You want an easy fix for this one?  Uninstall Google Desktop. What’s going on with this?  Google Desktop installs something called a Winlogon notification which is causing the hang-up.  Google Desktop somehow gets broken and causes Winlogon to hang while sending the notification that you’re logging on; Control+Alt+Delete interrupts Winlogon, and causes it to abort the notification sending.  If you check the system event logs, you’ll see evidence of this clearly spelled out, though Google Desktop won’t be mentioned as the culprit.