return os to original factory setting

I am running a Dell inspiron 15r with an Intel core i5. when the laptop was purchased, it came with windows 8 installed and it worked properly. A short time later, I purchased an Office download. Everything was still functional at this time. Recently, I was looking for Windows Media Center. From a friend, I received a copy of windows 8.1 pro with WMC. The activation key was apparently not functional. From this I realized it was not a valid license. At this time, I simply want my system to revert to the OS with which it was originally installed. thanks for your help