I answered the question about the Product Key already.
If Windows 7 came preinstalled on that PC then you own it for the life of the PC and only need to download and burn to DVD or write to flash stick from the tutorial I linked you to reinstall with the Product Key on the COA sticker. If it's retail OEM use the key on the packaging.
THere is no reason to throw away a perfectly good license you own for the life of the machine/OS because you didn't know how to write the latest official ISO to flash stick. I gave you the tool in Clean Reinstall - Factory OEM Windows 7
which has all the steps needed to get a perfect reinstall.