Using CreateInstall 4.

How can I check if a file exists at Program Folders\MySetup\Myprogram.exe?

For example,
you can use 'Source Code' command

macrox_setint( "mymacro", fileexist( macrox_do("#progfiles#\\MySetup\\Myprogram.exe")))


After that you can use macro 'mymacro'