install.js different behaviour Mozilla 1.4

I have developed a xpinstall that works until Mozilla 1.3: It 
uncompresses a package with an exe file, a dll file and a txt file. But 
it is uncompressed in a temporary directory and the exe does not find 
the dll and txt file, only in Mozilla 1.4.

Could anybody tell me if there is a way to solve this different 
behaviour in Mozilla 1.4?

Thanks in advance. The code is the following:
---------
var xpiDoc = "data.txt";
var xpiDll = "libsoh323.dll";
var err = initInstall("myApp", "myApp", "1.0.0");
var f = getFolder("Program");
setPackageFolder(f);
addFile(xpiDoc);
addFile(xpiDll);
var ignoreErr = execute("instala.exe", true);
if (err==SUCCESS)
	err = performInstall();
else
  	cancelInstall(err);