ISZ files are also known as ISO Zipped files. They are simply a compressed version of an ISO file. ISO files are an exact image of a CD or DVD. ISO and ISZ files are typically used when making backup copies of software. However, the images are essentially worthless until mounted. Once mounted, you can access the file like you would any disk, and install the contents of the image.
Download and install a disk image mounting program, such as Daemon Tools, SysProgs or SlySoft.com (see Resources).
Launch the mounting software.
Load the ISZ file into the program, and click "Mount."
Click "Start" and then click "Computer."
Find and double-click the file you just mounted to launch the installation. It will be listed as a new drive you didn't have before. For example, if you only had up to drive E before you mounted, you will now have a drive F holding the contents of your ISZ file.