When I go to install it says "Can't install on this computer".
It sounds like someone sold you the discs that came with their Mac, not a retail upgrade kit like you can buy for USD$20 here. The install discs that come with a given model of Mac often include software that was bundled with the Mac, not just a standard OS install. Because of that, they are locked to only install on the Mac model they came with.
By the way, your iMac7,1 (mid-2007), given enough RAM, can run OS X 10.11 El Capitan, which comes out Tuesday (Sept 30th 2015), and is free. But yes, you have to get it to Snow Leopard before the El Capitan installer will run, so you'll probably have to pay the $20 for a real Snow Leopard retail kit.