Can't be done.
A conversion to/from 32-64 bit requires a clean install.
You can export your files and settings using windows easy transfer and then import them back after the clean install.
Apps will have to be reinstalled.
And... you can reuse your original windows activation key which is 32/64 bit independent.
You can borrow any 32 bit dvd to do the job.
Is there a reason why you want 32 bit and not 64 bit?
I have done just such a conversion in order to be able to run a specific 16 bit program, but otherwise, there is not much reason to convert.