Well do you have the Windows 7 install media? If yes then just insert the DVD or USB stick which ever you have and boot from that to start the install. If you are using a OEM copy of Windows 7 and just replaced the hard drive and added a SSD your key should still work. If however you have replaced the motherboard you OEM version will no longer work as it was tied to the motherboard it was installed on. In that case you would need to buy a new copy. But if it was a retail version you will be OK.