I bought a new computer, how do i change my xbox sign in?

Game tag will not let change to my new name and it is the first time i have change it. Niot my computer it change it My xbox is staying on the old name and know i cant sign on

Okay, thanks for your help so far....here is my router and modem info

I have a 2wire router and

an OvisLink ADSL MODEM OV303R2

dude...you need to alter either the wan settings or the lan settings in your router. i cant remember which it is, but that's what you need to check

Okay but how do i do that......I know i do something on the computer but....i dont know where to go

dude. the ip address to your router is on the back of the router. go look at it and write it down. then go to your computer and get on the website address for your router. (type in the number you wrote down) then go to WAN settings and you should see your
MTU settings in there somewhere