Easiest way to find out if user has either Windows 7 or Vista (through telephone support)?

If you have to provide some initial troubleshooting support by phone [or email], and you don't have access to the user's PC itself, what is the easiest and most foolproof question one can ask of the user to find out if the 'dumb' user is using either Windows 7 or Windows Vista?

For example: determining if the user has either Windows XP or Windows Vista/7 is easy. Just ask the user if the button at the left bottom corner is (a) either square with the word 'Start' on it, or (b) it is a round button.

But how do you determine the difference between Vista and 7?

Edit: For all the existing answers the user has to type something, and do it correctly. Sometimes even that is already hard for a computer illiterate user. My XP example just requires looking. If it exists (although I am afraid it doesn't), I think a solution that is just based on something this is visually different between Vista and 7 would stand above all others. (Which makes Dan's suggestion to turn over the box and look at the label" not so stupid). Perhaps the small 'show desktop' rectangle at the right side of the task bar (was that present in Vista)?

Press CTRL + ALT + DEL it says so in the bottom and most people are aware of this shortcut (especially in corporate environments where people have to press CTRL + ALT + DEL to login).

vs

Press Win+Pause (if you can explain to people where those keys are).

For reference, here are screenshots of the resulting dialog under various versions of Windows:

Windows XP

Windows Vista

Windows 7

The quickest visual way to tell the difference between Windows Vista and 7 is the taskbar by the clock. In windows 7 there is a box that can be clicked to show the desktop. Vista:

Seven:

This might look different if a user has changed the theme or something weird, but if they can change a theme, hopefully they should be able to manage one of the other methods suggested.