WinXP Professional has it, but both people have to have it. I forgot where it's at, I think it's under control panel.
EDIT: Here is some info on it: "Use Remote Assistance to have a friend or computer expert co-worker walk you through a solution to your computer problem. Just use Windows Real-Time Client to invite an online contact or use e-mail to send a friend an invitation. With your permission, the expert can view your screen or even take control of your computer. All sessions are encrypted and can be password-protected. You can even chat online as you work through the problem."
It's under programs in the start menu.