so i built a new computer, and bought the windows 8.1 OEM disk from amazon. Windows 8.1 System Builder OEM DVD 64-Bit: Software

well when i type in the key it says its not vaild. i called microsoft, and after being on hold for 2 hours, the lady with broken english said it was amazons fault. i called them and they said they could replace it but it will take a week. is there a diffrence between the oem version and regular? why can i not use that version on my computer? is the oem version only to be used by big companys like dell/hp? wtf?