You need some form of traffic shaping/QoS (Quality of Service) mechanism.
The problem you have at the moment is that all machines are equal.
I suspect what you'd need to do is introduce a machine that sits between your others and the internet router and decides which traffic can get through.
Looking around there's some free software here that looks like it would do it:
It will take a separate machine, a bit of networking to get it configured though.
Can you get your broadband speed upped?