<p>Is there any way to use this kind of format in .Net (C#)?
I want to use the same skin format that uTorrent uses in my app, but i can't get the transparent background.
Any ideas?
Thanks for your time.</p>

The PixelFormat enumeration lists the formats of 'bitmaps' you can create in .Net, so you'd want PixelFormat.Format32bppArgb:
http://msdn.microsoft.com/en-us/library/system.drawing.imaging.pixelformat.aspx
http://msdn.microsoft.com/en-us/library/3z132tat.aspx
However I'm not entirely sure that the BMP file format supports transparency - so you would have to save the file as perhaps a PNG file instead.