Will not read install disc just spits it out

Just got an iMac from my dad. It is an older one but he never had any problems with it. I purchased the snow leopard upgrade and have been attempting to install it. The drive just sounds like it is trying to read it and spits it out. This was an issue when I was trying to burn files to a disk to save for my dad prior to the install, but after putting them back in a few times it ended up working. This is not the case with the install disk! I have tried multiple times. It did load once, said I had to restart my computer (which I did), after the restart it spit it out again and wouldn't read it. I am so frustrated! Not sure what else to do at this point... Any suggestions?