Win [typically] comes with a recovery partition, which you can (should) copy to a flash drive. It’s OEM, though; I haven’t found a product key. I’m sure it can be done, but it’s hard to find tutorials because MS insists that a VM is not the same machine that it’s running on, and you need a new license.
I was thinking about doing this (installing linux, then running Win8 in a VM), but I eventually decided (considering how often I use windows!) that it wasn’t worth the effort.