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.
I have 3 computers and my PS2 hooked up to a 4 port internet /network router. I want to have one cpu (WinXP) and my PS2 to use a high amount of bandwith because I use them for games and downloading. The other two (WinMe) I could have at a lower bandwith because they are used only for internet browsing. When I use my PS2 or my good cpu (WinXP), I have to directly connect to my cable modem to get the best connection to play. If I leave it as is shared, I get crappy ping and enormous lag and get booted from games and such. I am fairly computer literate but I cannot get it figured out. I have a LinkSys network/router (don't know the model # as I am at work when writing this). Any suggestions? Thanks!!!