If your daughter has Windows Vista, you can set the parental controls to allow login between specific hours.
Start -> Control Panel -> (All Control Panel Items) -> Parental controls -> Click on the user, turn on controls and specify the time limits to which she can log in.
I'm a little hazy if this is a Windows 7 feature or a Windows Vista feature (I'm running 7) -- but you can also install additional controls on the Parental Controls page if you scroll down to the bottom .. things like website filtering and activity monitoring.
If you're on XP, it's probably easier to enforce only the internet access via the router. First, create a Schedule first under Tools, then create the access policy. Make sure when you set the access policy you choose the MAC address of her computer since the IP can either change on its own if you use DHCP, or she may get smart and choose a manual IP address.