Most recent (k)ubuntu install CDs offer to run from CD at the boot menu. This is useful if you need to fix up your boot loader (usually grub).
If this used to work, but no longer does, you may have a hardware problem. Be methodical and isolate it:
-Can you choose memtest86 when you boot from CD? Does it run?
-Try another CD platter. Any better?
-Replace your CD drive. Any better?
-Open case, de-dust, reseat connectors and boards. Any better?
You get the idea.