No installer disk? No backup? 😱
And you "accidentally" edited a file inside /System? 😱
It's crucial that you have an installer disk (or Recovery Partition) for your Mac. Not only can you then reinstall the OS if you need to, but you can also boot to the installer and repair the disk, run Terminal, reset the password, etc, etc.
Without some other disk to boot off, you NEED to reinstall the OS. You can buy Snow Leopard disks from Apple very cheaply.