That sounds like you have some old hardware there. An old Pentium chip maybe ?
The cx8 cpu flag specifies that the cpu has the command cmpxchg8b (compare and exchange 8b) as part of its instruction set.
The pae cpu flag specifies that the cpu has physical address extension capability.
These commands below can be used to interogate various information about the processor from the command line.
sudo apt-get install dmidecode
You could install Ubuntu using the net boot non pae mini iso and then install all the packages required for mythbuntu after installing the core Ubuntu packages.
sudo lshw -C processor
sudo dmidecode -t processor
From forum member kanasnoob.
You could also download and try to install an older version of mythbuntu.
These may fix the pae issue but i am unsure about the cx8 issue as i have no hardware without that instruction.
Unforutunalty your hardware is now getting to the stage where it is considered obsolete and Ubuntu is not the only distro to turn its back on the non pae kernel.