Buy an external hard disk enclosure, an EMPTY one. Make sure it can take 2.5" laptop disks. Put your spare hard disk in it.
Get a piece of cloning software like has been said.
Attach the external drive to the laptop.
Clone (copy) your existing hard disk to this new one in the external enclosure. This should make it an exact copy.
Take out the old hard disk and put the spare hard disk in the laptop case.
Laptop should now boot from the new spare hard drive.
Keep the old hard disk for a while "just in case".