Limit bandwidth of a LAN



At my job we have 6 computers + 2 i use myself, on LAN under same router.
Those 6 PC's are an "Internet café", sometimes i get people that use facebook/skype whatever and everything goes fine, but some other times people comes to download stuff, software, music whatever.

The router doesn't prioritize the bandwidth, and gives everything to the person downloading and nothing to the rest, at the point the rest of the pc's won't even load a single site!

I need a way to limit the bandwidth of the Internet café PC's.
the ideal would be a software i can install into my pc and manage the rest from there via LAN.
If not i could survive installing a single program on each client...but that program needs to be either hidden (no system tray icon whatsoever) or password protected.

Please don't recommend me NETLIMITER that software isn't neither hidden nor password protected so any user can disable it with 2 clicks.