[mr.exodia]

I found a solution that is clean and only requires modification of a few registry keys on program installation to work... The solution that works on my pc (after binding CTRL+ALT+DEL to taskmgr.exe) is this:

Just add the DLL to your AppInit_DLLs and task manager will only open if you created the mutex named "green_light"
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows NT\CurrentVersion\Windows\AppInit_DLLs for 32 bit taskmgr.exe
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Windows\AppInit_DLLs for 64 bit taskmgr.exe

Attached source + dlls, fully tested and working on Win7 Ultimate x64.

edit: you might wanna read some topics about AppInit_DLLs if you're planning to run your software on Windows 8. Here it requires you to set some other registry keys as well. also added a small mutex tool so you can test it without writing much stuff