Hi,
I believe I tried the same route your attempting. I couldn't create an ISO on a USB flash drive from a XP re-installation disk, "Win to Flash". I did have a W8 key on a USB flash and installed from that. When I tried to register the OS got a message "OS was only for upgrades". Upon the next 4 or 5 times I booted up I got the message" Register your OS". After that no warning message. I am now getting W8 updates.
One time I transferred a XP OS from one computer to another and was told that I had 30 days to register. I did and no sweat. However W8 has only been in the NUC for 2 weeks. I could still get a hit but by then I should know how the NUC is working.
Would you please post a link as to how I can make an ISO of a XP re-installation disk and transfer it to a flash? TIA