She's probably resetting your password, you don't really need to crack or guess the password on a windows machine, just need a basic program that will boot off a floppy or CD that will reset all accounts.
I would do what Trotter suggested, by first setting the boot priority to Harddrive first, this way it prevent anything from booting off the CD-Rom or floppy. Then there should be two option for BIOS password, one for entering the setup and the other for booting up the machine. Put a password on for both, then lock your computer case.