There are two problems here:
The way that Process Explorer takes over from Task Manager is by adding a registry value:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\taskmgr.exe\Debugger
This means whenever you start an executable named 'taskmgr.exe', start whatever is specified in this value instead. So moving task manager to a different directory does not work.
Renaming taskmgr.exe to something else seems to break something
within Task Manager. That why you see an empty window. That's a
rather odd behavior, normally it is no problem to rename an
The renamed task manager behaves rather odd. Even if started from a medium integrity level, it automatically runs in high integrity level and it can not be killed even with local system privileges.
So for now, I don't see a way to have Process Explorer as default task manager and be able to run task manager at the same time.
The workaround is not too bad:
In Process Explorer, untick 'Options - Replace Task Manager', then run 'taskmgr.exe' and tick 'Options - Replace Task Manager' again.
Alternatively you can create a batch-file that removes that registry setting, starts task manager and puts the setting back in:
reg.exe delete "HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\taskmgr.exe" /v Debugger /f
reg.exe add "HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\taskmgr.exe" /v Debugger /t REG_SZ /d "C:\PATHTO\PROCEXP.EXE" /f
The only problem with this batch is that you need to run it as an elevated administrator, so when creating a shortcut for it, specify to run it as an administrator. As a normal user you cannot change those registry settings.