Computer Not Reading Discs

Whenever I insert a disc for a game, nothing happens unless I click on the shortcut. This may not sound like much of a problem, but It is if I want to re-install a game. The same thing happens with game demo DVDs. I've tried entering D:setup.exe. into the Run box, as a game instruction manual told me to do, but it comes up with an error message:"D refers to a location that is unavailable. It could be on a hard drive on this computer, or on a network. Check to make sure that the disc is properly inserted, or that you are connected to the internet or your network, and then try again. If it still cannot be located, the information might have been moved to a different location."In the manual (which happens to be for Freelancer, but that's not important), it says "type D:setup.exe (where D represents your CD-ROM drive)". I don't know whether D is my cd-rom drive or not, how do I find out? Or can I just click "enable autorun" or something similar somewhere, and will this solve the problem? Also, this may or may not be related, recently, the button on my keyboard which closes the disc tray has stopped working, I now have to manually push the tray in (there's no button on the computer).