May I point out to everyone, that you could try a manual installation. That means that you download the install disk ISO, for free from here: http://windows.microsoft.com/en-us/windows-8/upgrade-product-key-only and completley bypass the windows store.
Enter your product key, and then download the ISO, burn it to a DVD or use the Windows 7 USB download tool, so you can use a live USB to boot. Then BACK UP all of your home folder. That means going into your user name folder form the start, and then copying everything over to an external hard drive, or memory stick, or even a network location (people might be able to see your backups on a network location, so I would not recommend it, Also, if your PC can't boot, then the recovery CD is not going to give you any network connectivity if your PC cannot boot properly). This is the easiest way to backup, as you can see how quickly it is going, and how long it has left. Also go start, type recovery, (go settings if you are in windows 8) and then find the one in the control panel. Select create a system image in the left sidebar, and then select your location. Make sure you have enough free space, and if you do not, delete some files, or even put them on skydrive. When this has completed, I would also make sure that the windows 8 ISO lets you restore a system image, so if anything does go wrong, or you do not like windows 8.1, or if it does not work for you, you can restore your backup using the live USB. Also, I would then do a clean installation, delete your Windows partition, create a new one, and then install windows 8 to that. You will have to re install your programs, and copy your user folder back to the one on 8.1, but it means that the upgrade will work...
This has not been tested by me, but it probably will work.
Hope this helps
George
No, I've already tried this and in my case, it doesn't work...
What's your case?