Downgrade Vista from 64 bit to 32 bit.

Hey.
I recently bought a HP compaq 6715b running Vista business. When I went to install it recommended 64 bit, and after finishing i made the back up discs. However, It seems that Zonealarm, among other programs, does not work, and since I will not be using the notebook to be running programs requiring >4gb of RAM I wish to downgrade my system to 64bit.
I tried reinstalling with the discs hoping to get to the selection screen again, but no luck.

How can I downgrade?

Thanks in advance guys