Migrate from MDX to what?

Hi I've got an application (kind of CAD-like thing) using Managed Directx. I wan't to stop using this for two main reasons:

- Most users have to install a special dx9 add-on from MS to be able to use it.
- Mysteriously, arithmetic operations in other parts of the application lose accuracy when the graphics are running.

They say XNA is the way, but I'm concerned about these things:
- I want it to run on Windows 2000 or at least 2003.
- I'm afraid of having to replace my application's main form with a special XNA game object or whatever it is.
- It might be a huge amount of work to change the code

I've heard of SlimDX, but it looks like the default is a massive download that would double the already bloated size of my application. I'm a bit scared that I might not be able to cut that down to something reasonable.

How about using non-managed DX9 from .net? Is that practical? Possible?

Any solutions to these concerns?

The things I'm doing are fairly basic. Mostly dumb vertex buffers full of shaded or textured triangles, or lines. Frame rates are relatively low, it only renders when the user repositions the view or modifies the content.