PCs used to come with Windows installation disks. If you fucked up your computer, you could do a clean reinstall. This was the case up until about Windows Vista, I believe. Around that time they stopped shipping installation disks and put a rescue partition on the drive instead. The rescue partition only works if there's a recognizable Windows install on one of the other partitions. So you can't do a random write over the Windows partition and reinstall Windows. If you write over the whole drive, the rescue partition is gone too. If you create a backup disk, you run the risk of backing up the files that you want to destroy. So there is no way to securely reinstall the copy of Windows that you paid for (ie, with that license). I suspect that's not an accident.