Since you never told us what OS you are using, I'll assume it's Windows. If the both of you have a separate account in Windows(one account is yours and the other theirs) then put a password on your account. Once you do this, you can make all of your folders private. The only way your housemate can look in your files/folders is if they have the password to get into your account. If you want more info on how to do this, let me know.