You can easily modify windows 7 executable and change images inside executable & DLL files to have your own custom mod. Resource Hacker like tools makes images replacement task pretty much easier. However replace system files with your mod need slight different treatment.
If you are looking for easier way to replace Task Manager with your own modified executable then Task Manager Replacer v1.2 for Windows 7 right tool for you. Task Manager Replacer is a simple application that will enable you to change your task manager.
You just press the Manual button and select your Taskmgr.exe mod and your new task manager will be shown automatically. You can also restore default task manager.
Task Manager Replacer
If you want to download different Task Manager Mods then checkout this link.
Download Task Manager Replacer.