If you try to activate Windows 8 RTM you may receive the following error messages :
"Windows isn't activated", "Error Code : 0x8007007B" and "The filename, directory name, or volume label syntax is incorrect."
Basically change the Product Key as follows :
Open a command prompt window as ADMINISTRATOR and type
slmgr /ipk NEW_PRODUCT_KEY
The old product key will be replaced with the new one and Windows 8 RTM will be activated automatically.