Help with speed issues

Hi, I share a house and internet connection with 3 other guys and we are having issues. One of them says that his online game keeps lagging. Each time he said this is happening has been a time when I was on youtube or hulu. I didn't think that would be a problem. Is there a way to throttle my connection so it doesn't lag his game? I don't wanna cause trouble because he is getting annoyed and has already showed annoyence in paying what we are for it because it is a bundle deal. We have Comcast. I think the connection should be a 12 megabit connection. As far as I know this should be plenty, even with four people. I have Windows Vista home premium 32bit, but am going to be getting Windows 7 64 bit. Any help or advice would be much appreciated.

Thank you for your time.