Ok, one thing to keep in mind is that those numbers are in
KiloBITS not kilobytes.
As far as a Web browser is concerned you are only getting 511 kilobytes down and 76 kilobytes upload.
You said that there are around 5 PCs on the network? Remember that to surf the web comfortably, any PC really want's to have about 25 KB upstream so that they can send data to the sites they are requesting information from, basically if 3 or more computers are surfing the net that will choke it out really quickly, not to mention that if someone is uploading files, whether through file sharing or MSN or e-mail or what-have-you, that upstream will drop to insufficient really quick, giving the impression of slow internet performance. What's really happening in a case like this is that the web site that a PC on the network is trying to connect to is waiting for the requests to get to it before it can serve the proper info.
Choking the upload speeds can make the fastest connections seem like they're crawling.
On the other hand I'd still submit the
HJT logs from your brother's roomates computer.
Also, if your machine is the gateway to the internet, if he refuses to help you keep the network safe...disconnect him from it. There's no way in the world I'd allow someone to jeopardize my machine just for the sake of being headstrong...I find that pretty inconsiderate to be honest. Mybe a few days without the internet will change his mind.