Let us assume, you have the following in your script. The %MYVAR% variable is set to 0 and if you execute the script the Message Box displays 0.

Set Variable %MYVAR% to 0
Message Box "%MYVAR%" -- "Test"

But if you start your executable with the following command line, it will display "cool" instead of "0".

yourinstallername.exe /V MYVAR cool

The above sets %MYVAR% to cool (please do not add the %-sign around the variable name).

Does this help?