Programmatically replace transparent regions in an image with white fill?

Question

I've got a PNG image that I'm operating on via the System.Drawing API in .NET. It has large transparent regions, and I would like to replace the transparent regions with white fill--so that there are no transparent regions in the image. Easy enough in an image editing program... but so far I've had no success doing this in C#.

Can someone give me some pointers?