Recently Autodesk Product Support was asked about trying to install the Hydraflow apps from Civil 3D 2009 while the 2008 application was still installed on the system. While running the 2009 install, the user found that the Hydraflow applications were disabled in the Customization page.
Since the same Hydraflow applications were included in both the 2008 and 2009 Civil 3D, the best solution is to access the maintenance mode of the 2008 release through Add Remove Programs, and run the first option, Add or Remove Features. The second screen, Add/Remove Features will allow you to uncheck the Hydraflow Extensions and continuing through the wizard will then uninstall the Hydraflow from the 2008 release.
While running the Civil 3D 2009 install, the option to install the feature, Hydraflow, will then be enabled and can be installed.
Alison Gangler, Autodesk Product Support
Alison Gangler
Install Engineer/PDG-AEC Product Group
Autodesk, Inc.