How to create a cross-platform DLL in .net

I have a interesting problem: Where I work we've built a home-grown ERP system in VB6 that we are slowly moving over into vb.net. There are some projects have are in .net: we have a hand-held C# project that uses a web service to talk to our database, I've built some reporting screens using Crystal and some smaller maintenance screens.

Well as we have been plotting the conversion out, we want to have a way to separate our business logic and UI so that the UI can be a win/web form or a Smart Device project. Is this even possible? I try to reference the DLL in a test I have and it gives me this error when trying to debug using a emulator

Deployment and/or registration failed with error: 0x8973190e. Error writing file   '%csidl_program_files%\smartdeviceproject1\system.windows.forms.dll'. Error 0x80070070: There is not enough space on the disk.

I'm not sure what it's doing... I take my DLL out and it works fine. Does anyone know of a way I can create a DLL that can target all of these UI without may changes?

This post here helped me alot. Using a linked projects with conditional complation would seem to work in my case.