click on start. computer, click on system properties (top , 2nd from left)
click on change product key (bottom right in blue)
Enter new code and go to windows updates for validation.
You need to pick the windows validation tool first then let it update the rest.This will sort piracy from genuine codes.
If the disk media was seperate from the code this will let microsoft decide if the licence has been paid for. There neednt be any link between the licence and the code origin(disk,etc),
M/S only care you have paid to use it..