Did you sign out completely? I share this computer with my son who is also registered. At work I share the communal telephone computer with another Doper. There are many Dopers who share computers.
You need to go to the main forum page. Here: http://boards.straightdope.com/sdmb/
Way at the bottom, not in an obvious place, over there at the right hand corner past the last forum listed (The BBQ Pit) there are 3 hyperlinks: Log out, Mark All Forums Read, Forum Leaders.
Click on the Log Out link. You should get a message that says something like “Thank you for logging out. All cookies have been cleared.” Your wife should be able to register after you get this message.
It’s the same rigamarole. Sign in, get the e-mail, change password-- yadada, yadada.
The real problems starts when you forget to log out after your wife has used the computer and make a wonderful, insightful post under your wife’s username.