Did you recently use any tricks or tweaks to speed up shut down time ? Mostly this is caused due to faulty writes to restore points during improper shutting down. Really Sucks. Try opening up System Restore in safe mode and restore to the same point. [Hit F8 during boot]. Ive had this many times, and restoring in SafeMode worked for me.

Read http://support.microsoft.com/kb/q283096/ for further details.