windows - home network - user authentication and internet restriction

I have a home network (using a WRT54GL). I have 1 wired desktop, 4 wireless laptop/desktops and a Tablet (Android).

I want to be able to restrict internet access based on specific users. Some of the laptops/desktops are used by myself, others by my kids. Restricting by IP/MAC won't work.

Ultimately, I'd like to do /home drives, file shares, printers, etc. All the current machines (except the tablet) are Windows 7 or XP (pro).

The proxy approach - outlined here - is one option for the internet restriction.

I can setup a small server box if needed (likely using older hardware, trying not to sepnd a lot of money).

What's my options? Suggestions?