I've recently installed Windows 8 Pro in the first partition and in the third partition there was Ubuntu 12.10. After the installation, I could not boot any other partition. So I decided to uninstall Windows 8 Pro, and then, after recovering Ubuntu or Debian, putting it on a Virtual Machine. Can I reuse my product key in that installation? How do I uninstall Windows 8 Pro from my computer? Thanks in advance, Sho.
Yes, you can reuse the product key when re-installing on the same machine. As long as you have only one active installation, all is good. If you create a new virtual installation with the upgrade edition, you'll still have to either install a previous version first, or activate win8 manually by contacting MS over the phone.
For the booting problem, the windows 8 boot loader won't be able to detect non-windows bootable partitions (without 3rd party boot loader, like EasyBCD), but it will work easier the other way around - to have Grub2 be the boot loader with options to start Linux or Win8. If you haven't removed or reinstalled anything yet, it should work by reloading grub2. Some machines have secure boot options that may need to be disabled thru bios.