Does your server have Exchange System Manager installed?
I escalated (and it's fixed in 8 SP 3 - which isn't available) an issue exactly like this. The installer checks for the presence of a file which is part of Exchange System Manager, so if ESM isn't installed, the install fails.
Workaround (for now) is to copy exadmin.dll to the bin folder from an existing Exchange 2003 server which does have ESM installed, or temporarily install ESM.
Hope that helps,
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