Can't open Task Manager
- Download sysinternals suite from microsoft technet and unpack.
- Find and run
procexp.exe. - Click in Options menu on Replace Task Manager and than Restore Task Manager.
- Click Ctrl+Alt+Del or on taskbar Run Taskmanager. Settings are restored and MS Taskmanager works fine. Because of my habit I can´t work without sysinternals.
Process explorer by Sysinternals is one solution but there is another.
Use the built in System File Checker tool in Windows 7
Open a run command box or elevated command prompt and type in
sfc /scannow
it will now scan all your Windows system files and replace any that are missing or corrupt.
Source of Information
I had the same problem and I could not even fix it with process explorer. The culprit was the value "Debugger" under "HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\taskmgr.exe". Some older installation of Process Explorer which had not been properly uninstalled had registered itself as the debugger of taskmgr.exe by storing its path in there, but that path did not exist anymore, so windows would refuse to run taskmgr.exe because it could not find the debugger for it. Deleting the "Debugger" value fixed the problem.