If HTC XButton (Task Manager) is pre-installed on your HTC device but you wish to use a 3rd party Task Manager instead (e.g. Spb Pocket Plus), you can actually disable XButton fully by following these steps:
1) Go to Settings=>System=>Task Manager
2) Go to "Buttons" tab and uncheck all checkboxes.
3) Use MemMaid and go to "Startup Services".
4) Find "TaskManager" and change it to "Manual Start".
5) Soft Reset
If you do not go through the last 2 steps, HTC XButton may still appear in the taskbar unexpectedly when you rotate the screen (e.g. slide out keyboard) in the today screen. Anyway, it's always good to disallow loading of an unused application during the boot sequence.