Don't worry about the old disc, you don't need it.
You are in the perfect position to do a clean install. Just follow this excellent tuorial which has helped many thousands to attain a perfect install of Windows 7.
Clean Reinstall - Factory OEM Windows 7
You will see that you need to download the latest ISO file for your licensed version of Windows 7 from the authorised source, so you don't need your old disc!. The tut needs no additions by me but I would just emphasise that it is very imortant to first read the whole thing carefully a couple of times and if there is the slightest part that you don't understand please ask here before proceeding. It is very easy to guess a part and that could cause you to have to start all over again. Much better to ask first. It is important to not miss bits out or to try to bypass something.
Good luck!
Edit: Don't forget to le us know how you get on!