EucleX :
RobCherry :
yes you will need to re-install windows.
Wait, does that mean I lose all my files? Would I need a new windows key aswell?
You could you know...back up your files. But yes you do need to re-install Windows. Sometimes you can get away with not reinstalling if you're swapping the motherboard out..but its better to do a clean install in most cases anyway.
When you're going to reuse the same hard drive that has still on it, you need to delete the partition with everything on it and format the drive so its empty.
The Windows CD key is tied to the motherboard..so if your key is an OEM key (dell/hp) it may/may not work as you would need to active windows again.