All this net-spy stuff reminds me of reading a diary and the trust issues to go with. Just stick the computer in the family room, do it openly. Or orient it in her room so you can see what's going on with the door open. And the door will be open when the computer is on. Because it's your house and your rules.
Get in her face (have mom do it or be there) and say "your grades are slipping and we want to know why, so we're auditing your study habits."
If she has a cellphone, beware locking the computer can drive her to using it... which is harder to control.