Open cmd(command prompt) with administrator rights
2) Type slmgr.vbs –ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx (x should be replaced by your 25 digit product key)
3) Hit enter
4) Type slmgr.vbs –ato (This will change the product key)
5) Hit enter
Another method for fixing activation error 0xC004F074
1) Start -> Run ->Type Slui 3 (Slui 4 for activating by phone)
2) Enter Product key
3) Click Activate