Are you running XP through Parallels or using Bootcamp?
If using Parallels, Parallels Desktop>Preferences>Keyboard>Mouse Right Click and then check only the box or boxes that you wish to press in conjunction with the click to create a "right click" in XP.
Don't know how to do it in bootcamp. As a new Mac user I find it bloody annoying that apple don't provide two mouse buttons. It's one of the few areas where Window boxes have a superior design function in my opinion!