Copy the assembly HelloworldTask.dll to the Windows Application bin folder or the application extensions folder:
Location 1: <Install Dir>\ArcGIS\Mobile10.0\bin
Location 2: C:\Documents and Settings\All Users\Application Data\ESRI\ArcGIS Mobile\Extensions\ (for Windows XP:)
C:\ProgramData\ESRI\ArcGIS Mobile\Extensions\ (for Windows Vista or 7)
notice that the mobile installer does not create the extensions folder, you need to manually create this folder.
Good job on doing this custom task! 🙂
The solution would be to delete or move the DLL file from the Extensions folder.