cdrom dont read

18 different cdroms cant all be bad. im doing somthing wrong here. icant get cdrom to read. on install of new o/s, it goes to read cdrom and it says unable to initalize kernel heap. im taking it as not reading cdrom, right? i have changed cables, cdroms, ect. ihave checked setup to make sure cdrom is bootable. its a compaq deskpro. i had it working once and it hung up, rebooted and it kept going to safe mode and rebooting. i have to be forgetting somthing. please help. thanks