I second samtregar's suggestion.
However, to address the specific issue you raised, try compiling the dll using the __stdcall calling convention. I think that's as simple as using the '/Gz' switch if you're compiling from the command line. (I'm sure the same option is readily available if you're compiling using the IDE.)

If that doesn't help, then we might need to see the source code for the dll, and details of how you built it.

Cheers,
Rob