В cmd:
>start excel

Только Win+R или из окна командной строки тоже? В последнем случае выполни "where excel" и посмотри что оно тебе скажет.

Следующими шагами будет пытание assoc и ftype.

Не только в Path ведётся поиск

Finding an Application Executable
When the ShellExecuteEx function is called with the name of an executable file in its lpFile parameter, there are several places where the function looks for the file. We recommend registering your application in the App Paths registry subkey. Doing so avoids the need for applications to modify the system PATH environment variable.

The file is sought in the following locations:

The current working directory.
The Windows directory only (no subdirectories are searched).
The Windows\System32 directory.
Directories listed in the PATH environment variable.
Recommended: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\App Paths