For a while now, I've had Ubuntu runing on my desktop with all the free disk space, and I set up a Samba server so that my poor little Windows laptop can make use of the space. Now I have to reverse the roles, so the desktop is going to be running Windows XP and the laptop is going to be running Hoary. How can I set up the Linux laptop to use the disks on the Windows desktop?

If you install Windows XP on your desktop and create a shared folder that is gonna hold whatever files you want the laptop to access such as documents, music, movies, etc., then get your laptop setup, installing Samba on the laptop simply allows you to mount a windows share using:
, just read the Samba docs for 3.x. You simply connect to your WIndows XP machine (serername), give it the share name (sharename) and then where you want this to be mounted on the Linux laptop (/mnt/winshare) for example. Depending on the permissions and file system of the WIndows drive, you can then read + write. Pretty simple to get going.