I'm not familiar with PNUnit and don't know how it works, but the first thing I would try is force it to run as 32 bits by using the
corflags utility on its executable* (/32BIT+ switch). Beware, this modifies the assembly.
*The way .NET works is that the entry assembly (that is, the executable) decides the bitness of the assemblies to load. Any CPU jits to 64 bits on a x64 machine and to 32 on x86. If you run a Any CPU assembly on a x64 machine it will not be able to load an assembly built for x86.
Simone