I know on linux file permissions can be set so that those without permission's can't read them , but In windows I'm not so sure (though XP pro is supposed to allow you to do this). You could configure your firewall to deny your sister's comp acsess to yours, but she would need to have a static local ip.
One surefire way on windows is to unsahre the folder, then set up an FTP. You have to download then upload files when you want to edit them, but it is far more secure than windows file sharing (which I assume is pretty lax)
AGP won\'t fit in PCI-E... unless you use a really good hammer. ~Trotter