I have been moving a number of web applications from 64-bit Windows Server 2003 and IIS 6 running in 32-bit mode to 64-bit Windows Server 2008 and IIS 7. During testing we discovered the following COM exception when trying to pre-populate an FDF form using FDF toolkit 6:
0x80040154: Retrieving the COM class factory failed.
It turned out that FDF Toolkit 6 has been compiled only for a 32-bit OS. Thus, in order to make it work with IIS 7, you will need to register FDF Toolkit using the 32-bit version of regsvr32.exe. You will also need to enable 32-bit applications in your application pool.
Happy coding!