If your key is for Windows 8 (not 8.1), you can still install 8.1 using a generic setup key ( GCRJD-8NW9H-F2CDX-CCM8D-9D6T9 for Pro) and then change to your retail key after installation.
Without extended security updates (ESU), it is for internet-connected PCs. Use it offline or in a virtual machine only. Windows 8.1 Pro ISO Download 64 bit and 32 bit Updated