Updated on 11/20/2007 by Wrobinson:

You need a 64-bit machine to run the Active Directory and Domain Preparation commands from. One option is running these commands using a 32-bit version of Exchange and while this version is not supported in a production environment, in theory the schema changes should be identical. To remain in good graces with support, I would obtain a statement of support for doing so prior to executing it. Otherwise, you can introduce a 64-bit domain controller into the same site as the schema master and run the commands from there. The 64-bit DC can then be removed from the site demoting it, removing it from the domain, taking it offline and removing any DNS records for it; if there is any residue leftover, it can be removed using Ntdsutil.

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What error messages do you receive? Is your DC a 2003 R2? Is your Exchange running on 2003 R2? Do you have your Active Directory Schema correct?
We really need more details of what messges you are seeing as I have installed several Exchange 2007 (64-bit) in enviroments that run 32-bit 2003, one even had a 2000 SBS server that we eventually decommissioned. So, more details on what you are doing is required to give a better answer.

dmw