I'm manually setting up Zencart 1.5.1 on my 3rd party hosted website 1&1.

I have the shop setup pretty much how I want it and figured I would test drive a paid transaction.

I can add items to my basket, I can view my basket, but when I click on "go to checkout" I get this error:

Multiple Choices

The document name you requested (/index.php) could not be found on this server. However, we found documents with names similar to the one you requested.
Available documents:

/index.html?main_page=checkout_shipping (common basename)
Please consider informing the owner of the referring page about the broken link.
I sell digital products and have the 'Free Shipping!' module enabled. I've also set the product to not require a shipping address as the item is virtual...

Any idea what's causing this?