I've added code to my application that logs whether it's running in 32 or 64 bit mode (example shown below, using Msgbox in place of my logging function).

Setting Run64BitRuntime to False to force 32 bit mode as Phil suggests works (Thanks!), but doesn't seem to stick; when I run Visual Studio again it reverts to True and hence 64 bit mode. However, having now installed both 32 and 64 bit versions of the Office 2010 driver the application runs either way.

' If the Integer Pointer size is 4 then its 32 Bit and 8 is 64 Bit - could also use an Environment property

Dim BitMode As String = If(IntPtr.Size = 8, "64", "32")

Dim Server As String = Environment.MachineName

Dim CPUCount As Integer = Environment.ProcessorCount

Msgbox (String.Format("Started at {0} in {1} bit mode on {2} with {3} CPU cores", Now(), BitMode, Server, CPUCount)