Method A:
- Go to Systems Preference.
- Select Accounts.
- Create a new account in her name.
- Log into that new account.
- Delete the old user's account.
Method B:
- If you have the install disks, erase the HD and do a clean install.
- If not, consider purchasing Tiger or Leopard and erasing the HD and doing a clean install.
Personally recommend Method B since it will provide you a clean environment.