in the end i used unrar.exe. it's ok, it works. i used nsExec.
but now i have another problem:
my script is mothing like this:
Page custom CD02 ValidateCD
Page custom CD03 ValidateCD
in the first instfiles page i'm installing the first cd. with the first custom function i ask the user to enter the 2nd cd and i install it with the second instfiles page and so on...
my problem is: how to make my scipt to recognize that the first (or second) cd has already been installed ? because every time i use instfiles it starts from the begining (decompressing files).