When trying to launch any of the Hydraflow applications from within Civil 3D 2010 program showed splash screen followed by "Invalid installation. Program ending" message.
This is caused by missing HHApps directory within user profile most likely caused by installer not creating thes files or by some unwanted work of some clean-up tool. In some cases resetting user profile was still unable to pupulate missing entries. To confirm if this is the case first go to following locations and check if Express, Hydrographs, StormSewer directories exist in the HHApps directory.
C:\Documents and Settings\<user>\Local Settings\Application Data\Autodesk\C3D 2010\enu\HHApps
In some cases it is enough to reinitiate secondary installer to restore these missing files while in some other cases repair installation helped resolve this issue.
If none of that worked you may also try manually copying missing directories and files. To resolve this issue copy HHApps directory from any working machine and paste it in the location mentioned above and then restart Civil 3D 2010.