I ran across this tool while doing some research. I watched a video on Channel 9 that had its author describing and demoing the tool. It looks like it has a pretty easy search functionality that spits out VB code for the structures and method signature. There are a few other functions of the program as well, all of which have to do with wrapping native calls into managed .NET environment.

P/Invoke Interop Assistant available on CodePlex. The tool helps with converting unmanaged C code to managed P/Invoke signatures and vice versa. Say goodbye to digging through random header files or MSDN documentation to find the right constants, structures and signatures. The P/Invoke Interop Assistant does a smarter translation for you using SAL (Source Code Annotation Language).