TaskMgr.exe won't run. How to restore permissions and functionality?

I have recently removed some "alternative" process manager reporting software and cannot get TaskMgr.exe to run now.

Observed:
C:\windows\system32> taskmgr.exe
hourglass appears and then disappears. No process information and no dialog.

Likewise for Start> Run> taskmgr.exe

If however, a parameter is given, it will pop up a Process Explorer dialog box describing USAGE: Process Explorer [/e][/t]...
however all these should be optional (no parameter required). With no parameter, nothing appears (as described above) but with a parameter, the Usage dialog box appears for ANY parameter, including the allowed parameters.

NOTE: I observed the properties and see that READ and Read & Execute are allowed, however all the users and permissions are greyed out. Other EXE files in the system32 directory can be changed for permissions, but not this one. How do I restore control of permissions? I suspect that may lead me to why it's not working.

Actually, I was trying several replacements, although Sysinternals happens to have been one of them.

So, you inspired me to re-install sysinternals. The trouble is, the replace/restore option does not work properly now. The help file reads:
"Select the Replace Task Manager entry under the Options menu to have Process Explorer execute instead of Task Manager when you launch Task Manager. ... After replacing Task Manager the menu item renames to Restore Task Manager and selecting it removes Process Explorer's association."

Well, that does not happen. It always stays at "replace task manager".

This returns me to my original conundrum about permissions.

So, after deleting all references in the registry to SYSINTERNALS and still having the problem, I found an entry for TASKMGR.EXE
Deleting HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\taskmgr.exe
seems to have done the trick. TaskMgr.exe launches now, as expected.