How do I let users design a custom GUI?

Question

Hi,

I'm looking for some advice/information on a program I want to develop. I have some experience programming (mostly) in C# and using the .Net framework.

In the program I want to develop, I want the user to be able to design a custom GUI to suit their needs by adding/removing some predefined controls (buttons mostly). These controls have certain properties that the user can set. In addition I want the user to be able to choose the location of the controls in the window. I also want the users to be able to store the GUI they've designed.

I was wondering if anybody could give me some advice on the best approach for this kind of program? what language would you use for the controls? how would you store and retrieve the data of the designed, custom gui? are there any specific aspects I should pay particular attention to?

any help would be much appreciated.