windows user control

You simply add the user control by customizing the toolbox.
Have a look here:
http://www.startvbdotnet.com/extras/user.asp[^]

Aside from your initial question... When a user control also implements business logic, there is no actual separation and makes the control look like the "magic pushbutton" pattern. Could be a good idea to separate but it's up to you.

Good luck!