I put a CentOS 7 live CD into a PC several years old, and ran it without trouble for several days. Then I told it to install itself to the hard drive. That took kind of a while (perhaps because the machine is behind a KVM switch with three others?), but eventually told me it had finished. I rebooted.

Given my druthers, I would now run Yumex, deleting as many as I
could of all the things I never use. Then I’d update from whatever version is on the CD to the current one.

But I get errors, trying to run Yumex or to install it (both as root), saying “Could not resolve host: mirrorlist.CentOS.org” There are some panels, or what look to be panels; but the display on them looks like it’s been chewed up and spat out by some monster in the machine.
(Until I get logged in, the whole screen looks like that, or maybe as if two tilings had crashed into each other at high speed.)

Any clues? Pretty please??