Err. I got the thread you were refering to confused with another one. Sorry.
If we're talking about Windows past and present, interfaces are typically drawn with GDI/GDI+.
Some applications (like Trillian, Winamp, etc) that support skins typically use raster images (bitmaps usually) and clip them and fit the pieces together in a manner similar to Web pages, yes.
My opinion is that GDI+ and vectors are the best way to draw interfaces.
But clipping raster images does give you the ability to add photographic detail to your interface that is hard to achieve with vectors. This is not always the best way to go though because you can't always tell exactly how they will appear on everyone's monitors
and video cards. Plus it's much more difficult to allow the user to change the color or tint of the interface and customize it in other manners. And you don't get true scaling.