Vista - Problem nesting controls

I'm fairly new to WPF and have been trying different things to learn.
However, I'm a bit stumped right now.

I created a default WPF application in VS2005 and the Feb CTP. I got rid of
the <grid></grid> in the Xaml file, it's pretty much an empty
<windows></windows> xaml file.

I went ahead and programmatically create a Grid control and then a
ScrollViewer control. I then call:

ScollViewerControl.Content = GridControl;

This seems to work just fine and displays correctly.

However, if I changes the XAML file to the following:

<windows ..... >
<ScrollViewer Name="svXamlControl" ScrollViewer.CanContentScroll="true">
</ScrollViewer>
</windows>

And then in my code I change it to call:

svXamlControl.Content = GridControl;

The window appears fine, but my grid control does not render! What am I
doing wrong?

The other question I have is do all user controls require an associated Xaml
file? What about cases where my control simply derives from a base control
(e.g. Grid) and just overrides a few members. Otherwise, visually, it's
identical. Do I still need an associated XAML file? If so, what would I put
in it? The base control's tags?

Many thanks,

Winthrop