Q - how limit one LAN port on router?

Hi all,

I have a ADSL modem connected to a Sitecom DC207 router which in turn is
connected to 3 PC's, one of which is old, turned on 24/7, and mostly used
for downloading stuff. At times it rather hogs the internet connection,
slowing down the connection the other PC's can make substantially. Can
anyone suggest a way to limit the share this machine gets of the total
internet connection? The downloading software in question does not offer a
possibility to limit it's downloading efort to a certain amount. Is there
perhaps a way to squeeze the relevant LAN port on the router?

Any ideas deeply appreciated,

thanks,

Bob