A simple test that will help is if you disable boot from the optical drive in your BIOS.
If the machine boots without problems then, I would suspect your drive is in need of a firmware upgrade (get this from the manufacturer's website and follow their instructions carefully... a botched firmware upgrade can "brick" your optical drive.)
Keep reading before you try any firmware upgrades, though...
If this test allows you to boot from the hard drive successfully, then re-enable the optical drive as the primary boot device and attempt to boot from a Live Disk. If boot is successful, shut the machine down to "cold iron", allow it a few seconds for the stored energies to bleed down, and then start it again without the Live Disk in the drive. (The drive may just need to reset itself on power down.)
If you go through all this without any changes in behavior of the optical drive, consider replacing it. It may be lugging down the system by reason of some fault or other. (Replacement is the best option in this case. Optical drives are not terribly expensive.)
You could consider trying to replace the drive yourself. Depending on the computer, it's either four screws, a set of removable drive rails, or pins that hold it in place in the drive bay. It ain't rocket science - just make sure the power's off, take your time, don't force any of the connectors, and you should be fine. Dell and certain other OEMs also make instructions available in an online service manual for most models. These can save headaches if one is unfamiliar with some of the fancier "tool-less" drive cages.
Good Hunting on this one. Let us know how you make out with it.