100% usage is hardly 'idle'....
DO you have all chipset/gpu drivers installed?
What does task manager show for the process(es) causing all of the cpu uage?
And what do you mean, 'bought my OS from E-Bay'?
From an uninfected computer, download the OS from Microsoft using their Media Creation tool...(you don't want someone else's bootleg/malware creation media that could be full of who knows what spyware, bitcoin mining applications, etc...)