The problem has been solved after I luckily noticed a seemingly unrelated post on microsoft.public.sqlserver.odbc . Through some combination of installing, uninstalling and deleting files, a key had been created in HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Setup\ExceptionComponents blocking the proper reinstallation of the DLL. Deleting the keys related to SQL Server from that key hive allowed the reinstallation to fix the problem.
Randy Hersom
President, Habilitation Software Inc.