When you create a new account for her, she shouldn't be able to look through your files on your account(Unless of course you create her new account as an Administrator).
Here's how(Set for limited access):
Start -> Settings -> Control Panel -> Users and Passwords
On the Users and Passwords window, make sure the "Users must enter a user name and password to use this computer" is checked, then hit "Add"
Fill in the "User Name"(You can fill in "Full Name" or "Description" if you want), hit Next.
Enter the password, hit Next.
Hit "Other" then under the drop down, select "Guest", hit Finish.