Slow-down Issues!!!

Hello, I recently registered here just to make this post.

I'm just going to get to the point instantly, my computer seems to screw the network over. For some background, my dad and one of my sisters plays CoD an hour or two each day. Having said that, the issue shows a LOT. Say my dad is currently in a CoD match, and then say while he is playing, I watch some YouTube videos. Me watching a youtube video will make the 360 freak out and lag so much the same will be made nearly unplayable.

My problem isn't only with the 360 though, I make my sister's computer connection slow down a considerable amount, such as buffering youtube videos won't be an irregular encounter.

Now, this happens regardless if any youtube video isn't currently buffering, keeping Google Chrome open will still make her connection slow down and make the xbox connection terribly laggy.

The odd part of this is that my connection doesn't seem to slow down one bit.

What do you guys think? What should I do?

This problem has been going on for more than 6 months, if you need me to do anything, I am very willing if it does not have large chances of screwing anything on my computer up.

I can run any diagnostic program and post the results, and if you need to, we can use TeamViewer and I'll just let you do whatever you desire on my computer, except malicious stuff. And if you also want, I can get diagnostic info from my sister and/or dad, if that will help you evaluate/eliminate the problem.

One last thing, my router does not come close to capping at max "capacity" when I watch YT videos and my dad or sister is doing whatever. Also, if you want to chat over some IRC channel, Steam, or any other non-derpy chatting platform, then we can work something out.

P.S. sorry about this post being REALLY, REALLY long.

P.P.S. last month my network card failed (I was having the same issue then), got a new one, and I'm still having this issue, so you might want to look into other things before the network card

P.P.S.S The router we have is the Belkin N+ or Belkin N++, something along those lines. The modem we have is a simple Road Runner modem that they just hand out to their customers, it is also around 5.5 to 7 years old