Test if you have a space in some folder in your path to the project.
Example: D:\Lab\QtProjects\MyNotepad 1.0

This make that cannot recognize "windres"

Do the follow:
-Delete the pro.user file in your project root directory
-rename without spaces (Ex: D:\Lab\QtProjects\MyNotepad-1.0)
-in Qt Creator -> Open Project -> mynotepad.pro (following the example)
Compile and Voilé.... All ok!!

Try...