As above, go to Control Panel, User Accounts and create a new 'Limited' account & password for her to use. Whilst you're at it create a password for your existing account, or if there's already one, change it to something only you know.
Then, using your existing account, download and install something like Blue Coat's K9 Web Protection software (google it). It's free content filtering software for tinternet, very customisable and very tough to circumvent from the 'limited' account you have just created for her.
If the AntiVirus / Firewall software on it (you did get A/V & firewall software didn't you?) has the option – password protect that as well. Stops the little darlings turning off the stuff that is preventing them from downloading or executing the latest trojans. 🙂
If you need free AV Firewall options that include password protection then Comodo seems to do the job.