There's a couple of ways to do this using build-in encryption in Windows:
- Full disc encryption
- Folder encryption
This will keep the entire computer encrypted until she were to log in.
This is where a folder is encrypted, and files dropped into this folder will be encrypted.
Windows has built in encryption of both kinds, but it's important to follow Microsoft's instructions and save the certificate. If she loses the certificate, the data is lost. So she should put it in a safe place. Her hardware may have encryption as well.
Another option that Leo prefers is called TrueCrypt. It's open source and works really well.