You can reinstall Win 7 using the upgrade disc.
Put the disc in the DVD drive and open the setup file from the existing Win 7 windows Explorer - rather than let the DVD auto run.
Select the option to install on your C: drive and select the format option.
Wth Windows 7 it first checks that you have a valid Windows OS before the install procedure - hence by opening the set up from within your existing Win 7 (provided it has been activated with Microsoft) it treats it as an upgrade and will activate OK.
You will of course first have to back up any data on your C: drive to another partition or external drive - and reinstall your programs again because the C: drive will be wiped clean of previous files.