What version of Powershell are you using?

Just typing "notepad" is enough to launch it. The problems with powershell and executables only really come in to play when argument passing, where some escaping is necessary.

The only reason to use iex is if the thing you're trying to execute is not directly the name of the process. So you need & for `&'notepad'` and `&$string_variable` but not `notepad`