three2tango, you can make your screensaver password protected so that only you can use your computer and she can't use it while you are gone.
Here's how to try it (If you use a PC running Windows):
Go to Start > Settings > Control Panel
Double-click Display > Click the Screensaver tab > Set a screensaver (if you don't already have one) > check the box next to "On resume, password protect" and Click the OK button at the bottom of the dialog box.
If I am correct, the computer will automatically use the password you use to logon to Windows
I hope this helps!