512MB is not a lot but it'll work ( 384MB is the minimum for a GUI ).
A P3 won't break any speed records but it should run.
Your bios should have the option to boot from CD, double check your boot settings and try booting it with all hard drives unplugged.
Have you done a media check to verify the CD is good? What version of ubuntu are you trying to use?
xubuntu, damnsmalllinux and slax may run better than the standard ubuntu.