That's what it sounds like you want, and that won't come cheap on Windows. There are some Linux distributions designed to function as routers that'll do it, which would probably be your best option.

Lately Linksys has started adding a QoS (Quality of Service) function to their firmwares... Problem with it is, is that most of the devices don't have the processing power to be able to handle it. So, you could also buy a stand-alone router that has QoS/packet shaping capabilities and the processing power to do it, but those won't come cheap either.

If you've got an old 486 or Pentium box sitting around, you could probably set it up using one of those specialized Linux distributions, and that would be the best bang for the buck most likely. Though a stand alone router would likely use significantly less power and be easier on the utility bill.