If you can, backup any data from the old hard drive to another device(usb flash drive, external hard drive) or anything that would possibly fit onto the SSD. It's then best to format the old HD.
If you have a laptop or a pre-built desktop(hp, dell, etc..) then the Windows license key will transfer over/re-activate anytime you re-install on Windows(as long as it's the same version of Windows that originally came with the system). If it's a custom built system, you'll need to re-enter your license key to activate Windows again.
I have also seen instances where performing a clone still requires Windows activation.