If i downgraded from windows vista 64bit to vista 32 bit, how can i revert back to 64 bit vista?

My laptop came with vista 64 bit as its factory setting operating system but because i needed to use a software which was not compatible with 64 bit ier, i do hav downgraded to 32 bit. I do not have the cd for 64 bit becasue it was pre installed onto the laptop, however i do have the genuine product key etc sticker underneath the laptop. How can i go back to the settings which i had when i first purchased it?