You can normally contact Microsoft and they will go through the steps with you. You basically install everything as you would normal then it will start saying your copy in not valid (if it does not say that your ok, only happens with HDD change and mother board change). If it does say invalid you call them they will ask you to go through some steps and read some numbers. Then they will give you a huge sequence of numbers to enter and it will reregister that OS on that computer. Long as you have the product key number and be aggressive with them and don't let them tell you to buy a new one, it will be fine. Just had to do it myself with a upgrade to SSD.