I wasn't very clear in my first post here. You don't have to re-install anything here. What I meant originally was:

• run the %systemdrive%\Windows\SysWoW64\odbcad32.exe command, and set up your DSN
• run the %systemdrive%\Windows\System32\odbcad32.exe command, and set up your DSN, *again*

Each of those corresponds with each platform. The first is the ODBC manager program for 32-bit, and the latter is for 64-bit. The DSNs for each version are stored separately (at least the system DSNs are, but I digress). After you've got your DSNs set up for both platforms, everything should just work.