In what way does it not accept your serial code? Does it say it's invalid? Try phone registration, it should solve your issue.
Press the Windows + R to open the Run dialog, type slui.exe 4, press Enter, and go to step 7 below.
Click on the drop down menu, select your location.
Call the toll free activation phone number.
Enter the ID numbers on your phone keypad when asked to do so over the phone.
NOTE: These numbers are generated by your PC and provide information about your product key.
Enter the confirmation ID numbers given to you over the phone into the "Windows Activation" wizard.
When finished, click/tap on Activate.
If activation was successful, then you are done.
If activation failed, then you will be told that...