Assuming the window is called "Window" and the place you want to store the data is a table called "Data";
Code:
Window.PositionChanged=function(sender,args)
Data.x,Data.y=Window:GetPosition()
end
Will store the window position to the Data table whenever the window is moved.
Then, when you load the data - just use the Data.x and Data.y to load in the position.
Assumption: You are saving the data at some point with;
Code:
Turbine.PluginData.Save( Turbine.DataScope.Character, "The Name of Your Plugin", Data );
And loading it with;
Code:
Data=Turbine.PluginData.Load( Turbine.DataScope.Character, "The Name of Your Plugin")
or similar!