Quote: "not just a bunch of programs piled on top of each other"
BIOS->Grub->linux kernel->(???ubuntu software???)->gnome
All computer programs are giant sandwiches!
[edit]
sooo, this is how I *think* you can set it up:
1stly, back up all your important information from ubuntu. Then, make sure you have a linux boot CD you can boot into. Then, install windows on its partition. (If you don't remember which partition it is... i dunno, google around)
Now, windows will be installed, but you can't boot into linux - this is because windows overwrote GRUB with its own bootloader. All that means is that you have to reinstall grub! I think you can do this from the ubuntu boot CD.
this looks like a good guide.

Tell me if there's a broken link to images in a thread I post, and I'll fix 'em.