The first question you have to ask yourself is, Will I ever want to restore Windows 7 on this computer? If no, then here is what I would do.
1. Get the $39 download: Buy Windows - Microsoft Windows
2. Create an .iso during the download. Windows 8 Upgrade Assistant - Download and Run
3. Burn a DVD or create a bootable flash drive from the .iso.
3. Do a Clean install using the DVD or flash drive. Delete all partitions and create a new one during the install that encompasses the entire disk.
If you have an external hard drive, do an image backup of the current entire drive first.