Deploying HDF5DotNet causes 'Could not load file...'

I am deploying the HDF5DotNet as part of my .NET forms application which is written in C#. I have two version of installer: 32 and 64 bit.
My installers are MSI's that include the HDF5DotNet.dll and the native modules:


I am setting the PATH environment variable to my install folder.

The deployment works very well on most of the standard operating systems: Win7 32, Win7 64, WinXP 32,.
However, I am experiencing a problem with WindowsXP Professional x64 edition.
The install goes fine but when my code tries to use the HDF5DotNet wrapper I get the following exception:

Could not load file or assembly "HDF5DotNet.dll" or one of its dependencies.

The only thing that fixes the problem is to install the pre-built 64-bit installer available at the HDF5DotNet home page:
There must be something in that installer that allows the HDF5DotNet wrapper to find the native dlls.
Has anybody seen this before? Or has anybody any suggestions what might be missing from my installer?

Many thanks, Graeme