the contents of disc can't be changed!!

[urbanswerve]

Hello im trying to reinstall my os x snow leopard that came with the computer, after finding out that the boot up screen would not go i had to apparently reinstall. now it's telling me to select the disc where you want to install and there is only my hard drive dmg showing. i click on it to install and it then tells me the contents of disc cant be changed, mac osx could not be installed. i dont know whats going on or if i even have any of my files left. how do i get it back to normal please help thank you.