Can the drive also read DVDs? If so, try one. If it can read a DVD disc (video or data), then the drive is defective. I've seen this in two laptops (but not Apple laptops).
If, however, it's only a CD-ROM, it doesn't hurt to try things such as cleaning the drive, but most likely it is a hardware defect and will need replacement.
But before we go that route .. put in a OS 9 or OS X boot disc and try booting from it (pushing down "c" when you hear the Mac sound when you start it up). If it boots from the disc, then it is probably a software problem (which may need an OS reinstall, clean from original CDs). If, however, it cannot boot from the disc (and you've tried different discs and different methods), then it is most likely a hardware problem (and may need replacement). I realize you said you can't do a clean install (it seems obvious since you can't use the drive right!).. but just try this before you dive in and get a whole new drive.
Sometimes the lens just becomes misaligned and cannot be fixed very easily .. (although it could be a host of other things as well). Good luck