You can do one of two things:

1) Make sure that every time you're done playing, you log out of your account (click Options next to your username, then Logout).

2) If you're using Windows XP, you can set up different users within XP, each having their own desktop, documents, and browser bookmarks/cookies. The cookies are the important thing, as that is what keeps you logged in as you. If you do this, you'll stay logged in as you, and she'll stay logged in as her. Then you just have to train her to log in to her XP user profile, and not yours.

To do option 2, go to Control Panel, and click Users. Select "Create a new user" or something like that (I'm not on XP right now). It should be pretty clear what to do from there.

Good luck!

You can set up a different account for a different email. It will automatically log you in unless you hit Options above the game window and select "Logout" when you finish playing, or your gf can hit this to log you out when she gets on.