Sometimes SHELL can't read a path, because a space is involved, like "c:\program files\" the way around that is to enclose it in quotes:
SHELL "c:\" + chr$(34) + "program files" + chr$(34) + "\myfile.exe"
Where chr$(34) represents the ASCII value for quote marks.
You can check out the _CONSOLE statement to keep the SHELL command window open in the WIKI.
You could try START as in SHELL _dontwait "START notepad.exe" START is usually implied in QB64 but it might be worth a try. Some Windows programs START will find, without including a path. Notepad is one of those types of programs. the .exe can be left off, as it is also implied.
Hope you get it working,
Pete