Although it's not automatic (less professional?) you could display a panel asking the user for their CDROM drive letter.
Then prepend their response to a string var that you use to play the AVIs.

Just a rough idea:

String AVImovie = " "; // 32 spaces.
String DriveLetter =" "; // A single space
String AVI01 = "://AviDir//Intro1.avi"
<Get user input to DriveLetter>
str_copy(AVImovie, ""); // set to null
str_cat(AVImovie,DriveLetter); // add the drive letter
str_cat(AVImovie, AVI01); add the path & filename.

play_moviefile(AVImovie);

I don't know if this will work....

But there has to be a better way...