Your friend can invite you to join the show via Remote Assistance, built into Windows XP. http://www.windowsnetworking.com/articles_tutorials/wxprmass.html tells more.
You didn't intend to spy on him without him knowing it, I assume? That's not the way to treat friends and, moreover, it's probably illegal.
Is there a special add-on or an application, possibly web based, that I could use to co-browse websites with my friend? So that I could see the contents of my friends screen, when he's surfing the web.