I am not sure about windows upgrade disks having just come back to windows after 5 years of strictly linux. Now I run Windows 8 as well as Linux. You can try booting that win 7 DVD and see if you get the option of an upgrade or a clean install. if you get both you are in luck. See if the clean install option will work. CAUTION: make sure you have a working backup of all your data first before proceeding with either option. You should have a back up anyway at all times!
P.S. Boot the DVD, don't insert it while windows is running and execute setup.exe. That will perform an upgrade. Windows 7 is pretty good at getting drivers for a lot of hardware. However before proceeding make sure you have the drivers you will need for your hardware to work in windows 7 (either 32 bit or 64 bit). If you have the CD for your motherboard check to see if it has win 7 drivers, if not go to your motherboard manufacturer's web site and download them. Especially be sure you have your network adapter's driver. That way you can connect to internet after installation. If you have an OEM machine go to the manufacturer's web site for drivers for win 7.