You can use the WebClient class in .NET to retrieve a form from a library as
a binary stream. From there you can do whatever you like with the stream.

Another option is to directly connect to the SharePoint database containing
the forms in SQL Server. Note: This is *not recommended* by Microsoft. You'd
have to do some investigation which database and/or tables you'd need to
access to retrieve the forms, but after you have that it's the same thing as
with the WebClient class, retrieve the BLOB for the form and then use it to
you likings.
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S.Y.M. Wong-A-Ton