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
executable.
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
start taskmgr.exe
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.
Process Hacker offers the ability to Start Task Manager in the Tools menu. If that would be OK with you, you might want to look for the same feature in PE. – JC2k8 – 2013-10-25T11:30:23.977