Upon rebooting the computer, I received a message that Data Execution Prevention had prevented the Userinit Logon Application, and after I closed that message, I was faced with just a black screen. I wasn't able to find any way to load Windows Explorer or anything else.
I have tried rebooting in both Safe Mode and in Directory Services Restore mode but in both cases, I'm told that windows failed to start.
With my other computer I downloaded PC Regedit and burnt it to a CD, and booted my computer from that disc. I was able to check the registry entry for HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\Userinit\C:\WINDOWS\system32\userinit.exe, and it appeared to be present and valid.
I don't know what else to do... I've reached the limit of my own knowledge. Please, can someone suggest a fix for this other than reformatting my computer?