Welcome to the Seven Forums, Ninchie.
One way would be to download an ISO image of your version of Windows 7 from Official Windows 7 SP1 ISO from Digital River « My Digital Life
and burn it to a Windows 7 Installer DVD and reinstall Windows.
You might have to use the "eicfg Removal Utility" in case your version is Windows 7, Home Basic 64-bit.
To activate you can use your existing Product Key.