The download is in the form of a .rar file. To open a .rar file you'll need either WinRAR (a commercial program) or one of the free compression/archive programs like 7zip.
You can get 7zip here: http://www.7-zip.org/
Get the version that fits your operating system....in other words, 32bit or 64bit depending on what you are using. Install 7zip.
Download and install ImgBurn. www.imgburn.com
Open the .rar file and move the DVDShrink 3.2.exe file into the existing DVDShrink folder in C:Program Files. (assuming you are using XP). This will overwrite the existing executable. Now, once you start up the program and are set to burn an ISO, you'll see an option to burn with ImgBurn instead of DVDDecrypter.