Any program that is running can hijack hotkeys and set them globally, ATI is just well known for hijacking Alt-F5.
Fire up your task manager and kill programs one by one. After each kill recheck hotkey in TC.
Of course I presume that you haven't set/deleted any hotkeys yourself in TC. If you don't know anymore open your wincmd.ini and check the section [shortcuts]. If you see an entry F5= there, delete it.
Icfu