Windows Vista old vista hard drive

Ok so this is the first real pull apart of a laptop i have done, my girlfriend recently upgrading and crossed over to the mac side! Leaving her old laptop dead, so i have stripped the main parts from it, upgrading other computers with it, well i only have a 120gb HD im my current laptop but with the space for an expansion, 2nd drive, so I thought i would give it ago, i did and all went great! works fine was given access to it rescued alot of old documents, picture files, but now i would like to boot from the vista which is installed on the second drive, how do i go about doing this?

 

Thanks for the reply, yes i undersatnd the term drivers and wasnt sure if it would cause problems, i have looked in the BIOS and to no avail! the second HD does not show up in the boot order?!? is this todo with the "drivers "?