Is it possible to tell who is using bandwidth on a LAN?

I was wondering if there was any well to actually tell who is using what bandwidth on a small network with a router. We have a, I think, 1.5 Meg connection or something in our flat and it's split between myself and 3 flatmates. The thing is, the internet is ridiculously slow sometimes and it seems to be when certain people are on.

I have a hunch as to who it is who is causing the slow internet speeds but since everyone is all pointing the fingers at eachother, I want to find out for sure who it is.

I was just wondering if there was a way to actually tell who it was that was using the bandwidth? (bear in mind that Im not very experienced so would need step by step instructions as to what to do!)

Thanks!

Jam it back in, in the dark.

Haha ok. I'll just murder my flatmates in their sleep then, seems the cheaper solution!

There's nowhere I can't reach.