I just downloaded IrfanView the other day for a different purpose and liked it very much. I was having trouble extracting some images from a zip file but IrfanView was able to open them up so I could save them direct to the computer.
As for the download I think if you go to the "download" page from http://www.irfanview.com/ just choose any of them that say "Download IrfanView" (not plugins unless you want those also).
But as far as making square thumbnails I've also been hearing about A Square Image lately, although I haven't tried it out myself: http://www.trivista.com/products/asquareimage/.