Custom materials - making an area of a texture reflect sunlight

[Przemek]

I'm making a custom texture for a building. I've noticed that some textures in HL2 and some mods have a specific texture applied on such blocks.

So my question is - how to do it?
My texture looks like this:

I want the windows part reflect sunlight...

Is it also possible to make, so that the wall part is concrete and the window part is glass?

Thanks in advance!