Centos 7/1503 -- Getting Error setting up base repository during installation

You have an error in the mirror URL (and extra x at x64) it should be http://mirrors.kernel.org/centos/7/os/x86_64.

Do not use LiveUSB creator, it doesn't work with this release of CentOS.

I had the same problem, I recreated the USB flash drive using dd under Linux ( dd bs=4M if=CentOS-7-x86_64-Everything-1503-01.iso of=/dev/sd[letter of your USB drive] ), and it worked like a charm.

If you do not have access to a Linux machine, you need to report this problem to LiveUSB-Creator developers and/or CentOS developers.

when you install centos 7 on network, you may config installation source.

when you input the url under the label 'On the network:', you'll see the check box already checked that means 'This URL refers to a mirror list' , disable that.