Q: OS X cannot be installed

Do a backup. Depending on what OS you are currently running either boot from the OS DVD (10.6) or the Recovery Volume (command -R on a restart) and use Disk Utility to Repair Disk and Repair Permissions. Run until no disk errors are found. Reboot and test.

Use the power button to shutdown the computer and then restart.

Do you have another computer (Mac or PC) or an external disc drive?

Which version of Windows is installed on it?

I was going to suggest that you attempt to recover your data using target disc mode, but I've just realised that most PCs don't have FireWire or Thunderbolt ports.

I believe that your Mac's disc drive is faulty, but it may still be possible to recover your data from it. If you can afford it, then I'd recommend that you pay for a professional to attempt to do this for you. Otherwise, try the following:

1. If you don't have an external USB disc drive, then buy one.
2. Connect the external disc drive (EDD) to your Mac.
3. Turn on your Mac while holding command-R to start OS X Recovery.
4. Open Disk Utility.
5. Partition the EDD according to the instructions in Apple Support article HT202796 (ignore the rest of the article).
6. Close Disk Utility.
7. Install OS X on the EDD.
8. After the installation has been completed and OS X has started (from the EDD), copy your files from your Mac's disc drive to your EDD using Finder. If your Mac's disc drive isn't visible in Finder, then I'd recommend that you attempt to recover your files using DiskWarrior.

(Phew!)