Disc or partition, same difference, you still need to tell the PC the correct place to look.
If you are having issues with the disc not reading that probably down to other settings.
You need to have the disc in the PC while its turning on and you also need to have the Bios set to read from the optical drive first.
You can not insert a disc once the pc has started and start doing an install, it wont work like that.
You can either go into the bios and set the drive/partition that has the W7 install on it as the primary or set the optical disc to read first and re install windows using the disc.
If you re install let it reformat the drive first. You will of course end up with a fresh install and any information on your PC such as pictures etc will be lost.
If this is an issue you could ask a friend to make you a live CD of W7 which you can run and hopefully access your drive info so you can back it up onto an external drive of some kind.
Personally I have never used a W7 live disc, the option just occurred to me and i looked it up, see here http://www.vikitech.com/8098/create-windows-7-live-cd