There are software tools to do things like that.
One free option could be Traffic Shaper XP.
That answer only works for one PC, or where that PC is the gateway to the Internet.
To do this for a number of PCs, you can configure queues on the router, if it supports this (Cisco can do this as can some others), or look at a device like Packeteer that is designed solely to do thi ssort of job.
It needs some degree of configuration, and if you know the IPs is not too difficult to achieve.
Try the community version of Smoothwall Express. It provides for traffic shaping capabilities for free and is strongly community supported.
In the IT trenches? So am I – read my IT-Trenches blog.