i saw this online and tried it earlier - but the key was missing.
after doing the slmgr re-arm and rebooting, the key has now appeared and i'll give it another shot.
clean install works, but only for about a day. then it asks for the key again and it wont accept the upgrade key (testing the regedit method people have mentioned, to see if that works)
try installing win7 (without a key) to the hdd, then erase the win7 install using the win8 disk, and continue... enter win8 key...
that is what I did.
well, i had a win7 tester install (just skipped the key entry page,) on the drive which i erased in the win8 installer, so maybe that is why i was not asked for the win7 key. I only entered the win8 key in the install and that is all.
try slmgr /xpr (that will tell you how long you have until you need to activate it.)
also google "slmrg.vbs" ... it will detail the options for slmgr...
just checked to win8 install I did yesterday. did not ask me for the win7 key.
but, I have the win8 in a separate SSD, and win7 on another SSD and i use the bios boot method (press f8 while in bios boot info screen) to get to the win8/win7 install. I dont use the windows "dual" boot screen.
TL;DR.. install win7 (skip key entry)(or any other qualifying OS)...after (the OS) win7 is installed, shut-down win7, then install win8 clean from the DVD (will need to enter the win8 key), by deleting the win7 install... that is what i did, and maybe that was why i was not asked for the win 7 key.
"slmgr /xpr" to check how long before you need to activate it.
BTW, yes this is way to complicated... but if an empty HDD/SSD, does not work for a clean install of win8... probably what you will need to do.