Microsoft Knowledge Base Article 301527 contains the following summary:
This article describes the steps to use to configure your Microsoft Windows XP or Microsoft Windows Server 2003-based computer to receive Remote Assistance offers.
The Remote Assistance tool can be configured to enable an expert user to initiate a Remote Assistance session by using the Offer Remote Assistance feature. This feature requires the computer of the expert user as well as the computer of the novice user (that the expert user is going to help) to be members of the same domain or members of trusted domains. Domains are used in corporate networks for security purposes, and a network administrator usually manages them. The Offer Remote Assistance feature is not a viable option for most home-based networks.
For additional information about Remote Assistance, click the following article number to view the article in the Microsoft Knowledge Base:
300546 Overview of Remote Assistance in Windows XP