Where Can My Users Download Virtual Files?

[DrByte]

I'm confused, how will registering at your site help fix my 400 error that I am having when users try to download a zip file from a product they purchased?

I'm guessing that kobra was simply pointing you to a normal Zen Cart site set up to demonstrate that the feature does in fact work just fine for most everyone else. Thus the problems you're experiencing are localized to your unique setup.
And/or he's trying to expose whether the problem might be something client-side as you said.