If you don't want to pay for a desktop sharing tool, don't despair. Tom Mighell over at Inter-Alia found a free one that allows you to share your desktop with up to 15 participants called Microsoft SharedView. It requires a software download for both the host and participants, and you'll need a Windows Live ID. Apparently, it includes desktop sharing and chat, but no audio.
SharedView isn't the only free tool out there. Yakkle offers instant messaging, voice and desktop sharing. You start by initiating an instant messaging session. Once you've connected, you can opt to share your desktop.
I haven't tried either of these out yet...if anyone else has, let us know what you think.