are you using different profiles to work on the machine? Is this XP?
given the following, You are using different profiles and you are using XP, you can set her as a non admin, and then grant her a disk quota that will only allow her "x" amount of space. I think this is possible on XP home, but I know it is on Pro. So this is what happenes. When you change her account to be a user instead of an admin, she will only have access to create and modify access on her profile and in program files, as well as any directory that is not locked down. From here, you only have to enforce a disk quota on her profile.
follow these to do that...