I am guessing that you are all paying for the cable line?  If you are the person who pays for the line you can place bandwidth restrictions on their ip addresses.  However, I have a 4 computer setup running on a 3 mb cable line and have no problems.  I know that all of my computers are not download 650mb movies from Kazaa but the speed stays pretty high up.  You might also download new drivers for you network card.   Also check you setting  you might be only running at 10baset instead of 100baset which would explain the slow down.  Let us know and more info would be great!

Lorne brings up a good point...  Some simple house cleaning could solve your problems.  Try removing spyware, updating your drivers, and power cycling you modem.  You may find that this take care of your problem.

Okay, here's some more details on the situation:

3 computers use XP, but the problem computer is running LINUX...

It's hard to get him to do any "house-cleaning", whether it has to do with his computer or the actual house.  He's not the most Linux saavy person using Linux, so he doesn't update a lot.  He's not using Kazaa or anything I've probably ever heard of before.  I have heard that he's used BearShare and EDonkey in the past, but I don't think that's what he's using anymore.  I would ask him to check whether he's running his card at 10 or 100, but he may not know how to check and he's not very receptive to questions like that anyway.  He thinks it's my fault somehow, even though I've shown him the logs as proof and shown him how our bandwidth "normalizes" when he turns his file-sharing off.

Our router is a 4 port Linksys and there is a 4 port switch connected to accommodate the extra person.  When I look in the outgoing router logs at the times when the bandwidth is really tied up, it looks something like this:

192.168.1.101 65.49.207.147 42540
192.168.1.101 24.64.142.167 6346
192.168.1.101 81.248.222.172 6346
192.168.1.101 141.156.174.208 6346
192.168.1.101 200.232.207.198 17640
192.168.1.101 217.85.33.162 6346
192.168.1.101 217.85.47.254 6346
192.168.1.101 217.85.42.43 6346
192.168.1.101 216.68.31.98 6346

Where 101 is him, and his are the only entries in the outgoing log.  About 10 new entries are added to the log every second.  The IP addresses that it shows are almost never the same.  In other words, every time a new one shows up, it's a different number, so I assume these are people pinging him.  The inbound logs are similar, but show the IP address of the pinging party followed by the port they're using.

I agree with ca3le guy, kick their asses hehehe.really I would probably start from the beginning,that is run only one(yours) PC first to benchmark the connection and see your avg. speed.Next I would bring the other units on line one at a time and test further until I found the bandwidth hog then I would kick their ass.You have been given some ideas that can pinpoint where the problem is,then you can fix the problem or restrict HOGS,doesn't matter what they are doing if it is using most of the BW all the time it needs to STOP.Of course I suppose you could split the connections and bring in a whole new drop as cheap as INTERNET service is now.I don't know about you but its not a problem I would want to live with.I take my surfing pretty serious so I would do something to fix the issue.Thats always been the problem with some room mates if they arn't drinkin your beer, or eating your food, or porkin your girlfriend, they are using using all the BW and messing up the porn tapes....lol!!!

  Microwave©
P.S. Good luck,let us know,tell your friends where the best test site is.
www.testmy.net