HTML and XML can probably be learned in a weekend, they're just simple mark-up tags. No logic or anything really needed like in an actual language.

Picking a language depends on what you want to do. I went from C++ to VB.NET to C# (as well as learning some PHP here and there and I knew XHTML/CSS before I knew C++)

Want to work with web applications? Learn PHP, CGI or PHP (You can use CGI in C++ so it may be easier for you)

Want to make VB like applications that have better performance/better syntax style? Go for C# or maybe even Java (though I think VB.NET outperforms Java so ignore the first commend).

Or learn a framework like wxWindows for C++ or OpenGL or DirectX.