64-bit conversion, Windows DLLs missing

  64-bit, add-in, excel, windows

We have an Excel Add-In dll which works correctly on a 32-bit Windows system with 32-bit Excel.
I have rebuilt the Add-In as a 64-bit dll.
64-bit Excel on a 64-bit Windows system reports the file as Invalid.
The Dependency Walker gives a huge list of missing DLLs, all of which appear to be part of Windows.
Why is this happening?

Source: Windows Questions