Microsoft has released a free Application and Desktop screen sharing software for Windows XP and Vista users codenamed “Tahiti” (via)

After you download and install the application, you can sign in with your Windows Live ID and start sharing the entire desktop or a particular application such as your web browser, Word, Excel, etc.

Tahiti is tightly integrated with MS Word so that every edit made by a participant to a document can be identified with the participant’s name and timestamp. Tahiti will also let you share files, presentations or images which other participants can download during the screen sharing session without needing to email it or upload it to a file sharing service.

You will need an invite code to sign up for the service which you can request in the comments of this post.