you've checked to make sure that you are permitted to distribute the program in this way - or perhaps you've written it yourself?
Either way, designing the site will be pretty straightforward. The main issue - if your program proves to be popular - will be bandwidth. Depending on the file-size you may find that you rapidly run foul of your web-hosting package bandwidth limit, and you'll be faced with sizeable bills. It's the price of success, and if you're offering a free download you may wish you hadn't started.