"Have a user account" or "Checkout Directly" ?

Help!

All that I want to do is to remove the "Have a user account" section from my checkout page. This is very confusing for my first time visitors so I want to simply provide a "Checkout Directly" button.

Having looked through the code I cannot find which file i need to edit to make these changes, or whether doing such a change will impact upon the rest of the site.

Your urgent advise would be most appreciated.

thanks,

Scott.