[FHwku]

1. you can log out of your account and uncheck the "remember this password" box.

If you use MSN:
2. If you haven't already downloaded MSN Internet Explorer 7, it has password-protected/user thingy.

If you have Windows:
3. Open you're control panel by clicking START, then Settings, control panel. or click on MY COMPUTER, then control panel.
it should have a USERS menu item. may be at the bottom.
it'll guide you through the anti-roomate process.
creating a user ID for you computer would allow her access to your pc, minus entry into your stuff.