In a full Active Directory environment, you can define the "Logon Hours" for a particular user (found under the [Account] tab in ADUC).

With local logon (no AD) I don't think you can restrict a user to specific times (unless you use logon scripts, which would get messy).

I do like the ability to use a Logon Trigger in 2005 though, in this context it seems a smarter solution.