Quick Registration & Login on a locked popup

[b65ran]

Hope I can explain it right! I am looking for a way to do this.

Is there any hack for me to have a popup window when someone visit the site and the window will not go away until you either login or register?

Popup window will have the following
1. Quick Registration (username+password+email) and
2. Login (username+password)

I searched through the whole forum here but no luck any hack, thread link or idea will help a lot!

thankx in advance