I've got my interpreter up and running - quite bug-free and stable for now - now I want to add some visual options to my language to play around.
What is a good GUI library easy to use and mainly easy to embed and "link" to my programming language?
What general rules do I have to follow?
I'm currently on XP with Microsoft Visual Studio 2010.
Depends on your language and it's properties. For example, if you can only expose C-style functions, then the default WinAPI supports this style. If you can do the whole C++ classes malarky, then you could do MFC, GDI+, WIC, etc.
Ultimately, if you want something easy to embed, it's simpler to write good embedding code than worry about what you're going to embed.
In various posts I have seen regarding UI Libraries QT and WxWidgets seem to be popular.