Make sure that the Disc you are trying to read is working perfectly on other computers.And that your optical drive is working flawalessly.
If yes ,then Please follow these steps and post the results ,
1.Try to Force The System to Boot from the CD/DVD only. i.e. Choose the Boot From : CD/DVD in the bios boot-menu.
2.Reset your Bios Settings back to default/original and try step 1.
3.Disconnect all Extra devices from the system that are not required. Like any External Graphics Card , Lan Card, etc. I have heard taht certain types of devices (specially some GPUs) , interfere with Bootable Discs.
Btw, are you using IDE or Sata drvies ??? There can be an issue in the jumper setting as well.
If you are using IDE drive then make sure that in the BIOS settings , the IDE auto-detect option is turned on o/w it won't take in bootable discs t boot-up but it will work fine for the OS.