I purchased a Dell laptop with windows 8 pre-installed. Then I upgraded it to windows 8.1 by Windows App Store, everything is fine. Now accidentally I updated the product key (invalid one) and windows got deactivated. Now I want to activate the Windows 8.1, I have genuine Windows 8 OEM key (found using get_win8key program from BIOS). I don't want to Go by installing Windows 8, then upgrading it windows 8.1 then installing all my software again as suggested by Dell. As at the time of upgrade It didn't ask for product key and I assume that program read the key from BIOS itself and Windows 8.1 is activated, so there should be equivalent key to be used to activate windows 8.1 again. Please suggest.
From our experience with Windows 8 upgrades, you're right out of luck. If your Windows 8.1 becomes inactivated for any reason, you can only use an 8.1 key to activate it. The keys are (intentionally) different and incompatible.
No, it didn't read an 8.1 key from anywhere. It just remembered it was activated and (because the Store is secure) it was allowed to stay that way. Upgrades via DVD seem to always finish up deactivated and needing a new key.
Your choices are (a) buy/acquire a new 8.1 key or (b) roundtrip through reinstalling 8 and upgrading through the Store. You can talk to Microsoft or your OEM supplier, but I wouldn't expect much help.
I have Asus Zenbook with pre-installed Windows 8 (normal, not pro). I installed clean Windows 8.1 and couldn't get it activated. It asked license key, which I didn't knew.
I was able to retrieve my licence key from BIOS (MSDM KEY) with WpKey util (pkeyui.exe). Windows 8.1 accepted this key and I was able to avoid all this "re-install Windows 8 and upgrade to 8.1".
There seems to be multiple ways of retrieving MSDM key: http://www.nextofwindows.com/how-to-retrieve-windows-8-oem-product-key-from-bios