Many problems caused by system file corrupion can be reversed doing what we call a "Registry Swap". We manually copy a backup of the registry over the current version of the registry to fix various issues relating to a corrupt startup. Here's how it's done.

• Backup the original files...
  rename c:\windows\system32\config\system -- system.bak
  rename  c:\windows\system32\config\software -- software.bak
  rename  c:\windows\system32\config\sam -- sam.bak
  rename  c:\windows\system32\config\security -- security.bak
  rename  c:\windows\system32\config\default -- default.bak
   
• Copy backups from the System Volume Information Folder...
  Look for folders named _restore{xxx}. Check the Created On date and find one created 2 dates prior to your current session. Look inside for a subfolder titled RPx (x is a number). RPx folders contain system restore points. Look in each RPx folder and open the Snapshot folder inside. Copy the following 5 files from the Snapshot folder to c:\tempreg
  
  _REGISTRY_USER_.DEFAULT
  _REGISTRY_MACHINE_SECURITY
  _REGISTRY_MACHINE_SOFTWARE
  _REGISTRY_MACHINE_SYSTEM
  _REGISTRY_MACHINE_SAM
   
• Rename the files...
  In c:\tempreg, rename each of the previous five files as DEFAULT, SECURITY, SOFTWARE, SYSTEM, and SAM respectively.
   
• Copy the renamed files to c:\windows\system32\config\
   
• Reboot & Test!