|If there is anything wrong with the setup file, you will get an error message when you try to run it. If you get no error message when running it, Avast should install okay.|
If you do get an error message, just download a fresh copy. There's no point in writing it to a CD anyway because it's free so you may just as well download it to your hard drive and install it from there. There's no point in saving the setup file because you can download a fresh copy as often as you want.