Why bother with booting it into a guest account by default? You said that guests don't use it very often; create a password-protected account for your wife, and a non-password (or, better, simple-password) protected account for guests.
When Windows 7 boots, it will present the two accounts as log-in options. Your wife can click on hers, enter her password, and be on her way. The guests that you want using the machine you can give the guest-account password to. The only account they'll be able to log in to will be the guest one.
There's no reason that I can see for making your wife go through the extra step of switching users every single time she uses the box.