If you have the Windows CD that either came with your computer (which is rare these days), or a backup was made onto cds then that's all you need. Most computers now just partition part of the hard drive off and put the software on there so you can refresh the computer in case of a bad virus or something like that, but of course if you change out hard drives that's gone. Anyways, pop in the hard drive, install the software of the cds and then you're good to go.

And Spoolin is right, if it's a Dell for example, and you use a Dell Windows cd, the install program will notice it's a Dell BIOS and just bypass the cd key part.