There are some reasons why attempting to install Snow Leopard can fail.
Suitability of your computer.
Valid installation DVD.
Hardware known to perform satisfactorily.
1. Is you iMac able to run Mac OS 10.6?
General requirements for 10.6 Snow Leopard
• Mac computer with an Intel processor
• 1GB of memory (though I'd recommend 2GB)
• 5GB of available disk space for the installation process.
• DVD drive for installation
2. Are you using the retail 10.6 DVD that looks like this? (A Grey disk will likely not work as they are hardware specfic.)
3. Does your optical drive play DVD's normally without any problem? If you think the drive is old and a bit dodgy, try using an external optical drive if you have access to one.
My Mac is able to run OS 10.6, I bought the software in the Apple store and my optical drive plays dvd's without any problems. I checked the memory, the harddisk, anything!
I've boot the Mac with the OS dvd and after a few minuts the installation stops because of another failure that it isn't possible to install some files.