You've gotta be kidding me...

[Pa_Kalsha]

If you're at all worried about losing your laptop, make sure that your data is backed up (you should be doing that regularly anyway, but most people don't). If you can't afford an external hard drive or USB thumb stick, writable/rewritable DVDs are pretty cheap.
It won't stop her trashing your machine, but it'll make it less of a disaster if she does.

Now, to stop her using your machine. There are three steps:

• Changing your main account password should be your first priority. Do that right now. I'll wait.


• Okay, the next step is a little trickier: change the admin password.
  (I'm assuming you're using Windows - I know nothing about macs, and slightly less about linux).
  It's possible that, instead of guessing your password, she simply booted into safe mode, and used the admin account to bypass the password.You'll have to search for steps on how to do that, but it's still really easy.
  
• Next, go into the BIOS settings and change the password.
  Now your computer needs a password before it even loads the operating system. Again, Google is your friend; search for the model of your laptop and how to access the BIOS settings.

Nothing here is 100% guaranteed; there are ways around all of it, but it should stop all but the most determined invader.