Home Network Bandwidth Control
Solution 1:
If your router doesn't support traffic shaping, you can install this on each computer:
Traffic Shaper XP is a free bandwidth limiter for Windows 2000, XP and 2003 Server. It combines high performance traffic shaping with the ease of use and flexibility to keep your network free of congestion.
Solution 2:
You're looking for QoS, assuming your router/gateway supports it. This should also be on Superuser.