Windows 8 doesn't boot: Error 0x0000000e

My Windows 8 does not boot. I get the error 0x0000000e. I already tried to fix the MBR with this tutorial:

Every time I run bcdedit.exe /create or bootrec.exe /rebuildbcd I get the error

Requested system device cannot be found

I can't reinstall Windows 8 because I only have the upgrade version and I don't have the key of my old Windows XP OEM version. So I can't activate Windows 8 if I would reinstall it.

Where do I go from here?