trying to make the checkout process NOT go to paypal

We are trying to setup our system so that when new members pay it looks just like your site here at http://wpmudev.org...

In other words, we do NOT want the user to "leave" our site and go to paypal for example to finalize payment, it doesn't look very professional to our clients.

How can we setup a "popup" registration from that looks similar to your site...

Thanks!

Chris