Rosetta ?
Do have Rosetta installed? Office 2004 is not a universal binary, it is PPC code, so it needs Rosetta (a PPC emulater of sorts) installed to run, and the installer must therefor be PPC code also.
Rosetta is not installed by default when you insatll Snow Leopard. It can be installed from the original DVD install disk - optional installations folder? Can't remember exactly what that was called, but look at the DVD when you put it in the drive, It's pretty obvious.
If Rosetta is already installed, perhaps your copy of the updater is corrupt?
Sorry, no other ideas now.