I have a Fujitsu notebook originally delivered with Windows 8 oem version. After many problems with Windows 8 I decided to install a Windows 7 Home Premium. I got a Reinstallation DVD of a Dell PC together with a sticker bearing a Product key number. The Windows 7 now works fine but I cannot activate the Windows as the systems tells me the Product Key of the sticker is not correct ( Error Code : 0xC004C008). What should I do now? Could I use an original product key of Windows 7 which I can buy in Internet? Now, I understand tha Windows version and product key must match. OEM Windows with OEM product key. How can I determine that the version of Windows 7 I installed is OEM or retail?