Springbok,
The dialog you are seeing gives you a chance to specify which types of files will be automatically opened in Irfanview when you click on them in Windows explorer. You don't have to select anything at this stage, you can do it from the properties dialog later.
There should be a button on the dialog to select just the image types Irfanview supports which is what I usually select. You can always change this later.
Once you have the main program installed, install the plugins. You will need some of these for some of the more obscure formats.
Adrian