Help

HI All,
I am developing an e-Commerce site, I am using eStore theme and eShop plugin here is the site
http://wowdesigns.in/projects/EC/

My question is
In the checkout page http://wowdesigns.in/projects/EC/shopping-cart/checkout/, I want just a login form to be displayed for already registered user, But here every time when the user wants to buy products he have to signup the form.

How can I give just a login form(just username and password fields) for the user who wants to buy second time(registered user)?

Kindly help

Thanks much,
Harsha