.NET file include from variable

hi,

I have a problem.
I want to make a page that shows different content on extensions.
e.g.
when address is default.aspx it shows the main page
when address is default.aspx?do=register then it shows the registering form
etc..

in php and that was quite easy
if request("do") = "register" then
inc_filename = "register.inc.asp"
else
inc_filename = "main.inc.asp"
end if

in .net I found an article where it explains that You have to use
<%@ Register TagPrefix="controlname" TagName="tagname" Src="file.ascx" %>
tag and control in the body.. but in this case it's not possible to change the source..

looking forward to Your help and guidence

R.