XBOX 360 Live - DNS Failure

Hi all,

I go through this every few months a for whatever reason, what worked one time will not work another time.

My computer was acting up on Friday. For instance, I'd go on Amazon.com, the page would load but when I went to sign in to Amazon that page said it could not connect. Same thing at all other sites I went to visit. It said something not right with my IP Address. So I clicked my network, clicked status and hit repair. All was fine...for me. Now it booted my Son off XBOX Live which has happened before. It finds the IP address but the DNS fails. Both were set to automatic so I went in and manually configured the DNS to what the computer said. When I did that, the IP would fail, this by the way he is running wirelessly (LINKSYS). So I connected him direct to the cable modem, bypassed the router (LYNKSYS) and that worked. It passed both IP and DNS but when when I unplugged him and rehooked my pc cord, my network went from 100 mbps to 12 mbps (which I don't know a thing about) and also I couldn't get online again but my son was running xbox wirelessly. How is that even possible? He's online but I can't get online?

Since I didn't need the computer last night, I left it so he could play. Today I tried to fix it, I can get back online but I couldn't get his xbox live to work, again! Even when I hooked it up directly to the modem like last night, the IP failed. I tried automatic, manual, anything I could think of I tried.

ANYONE HAVE ANY SUGGESTIONS??????? Also, if anyone knows how to get the mbps back to 100 as I am assuming I will now "network" slower. Although the only reason we have that is for his xbox to run wirelessly, I only have one computer so maybe it doesn't matter. Oh yeah, one other thing, the local area network is firewalled by Windows. When I try to turn it off, it won't let me. The on and off buttons are not enabled, the only thing it will let me do is click the button for "don't allow exceptions". I thought in the past, I was able to click the "off" button but I certainly can't now. Any suggestions on that as well?
Thanks !!!!!
Okay, sorry so long.