No real easy way to downgrade a 64bit OS to the 32bit OS version.
You're going to need to do a 32bit installation.
Have you thought about doing a dual boot setup? You could leave your Win7 64bit as your primary (or secondary) OS as then install the 32bit version in a seperate primary partition.
This guide mentions how to install Windows 7 and Vista in a dual boot setup. But it's essentially the same process to get a Win7 32bit OS installed as a 2nd OS on your system. http://www.sevenforums.com/tutorials/5166-dual-boot-installation-windows-7-vista.html