On Windows 7, if you go to Display > Set Custom text size (DPI), the "Use Windows XP Style DPI scaling" option is disabled (greyed out).
What do I need to do to enable this option?
I think you will find that "XP style DPI scaling" is only relevant if you have the aero interface active.
I've just confirmed on my machine - if I disable aero, the option becomes unavailable and when I then reactivate aero this reactivates the option.
If the aero interface is deactive you will effectively be using XP style scaling anyway because you are using the XP style interface.