How do I keep my melted candy windows in my gingerbread house from cracking?
Using melted hard candy, you are at the mercy of the ingredients of the hard candy manufacturer.  I like to make my own sugar glass.  Here is my recipe for sugar glass:


1 C Water
1.75 C Sugar
.5 C Corn Syrup - the clearer the better
.25 t Cream of Tartar 


I always supplement it with cream of tartar and corn syrup.  The cream of tartar stabilizes the mixture and the corn syrup helps the glass stay clear.  Sucrose (table sugar) is a disaccharide (two sugars) of glucose and fructose.  Most cracking occurs when when there is a structural defect due to crystallization.  Applying acid (cream of tartar) causes the sucrose to break down into its component sugars and stabilize.  

Also remember that heat and humidity are the enemy of sugar glass.  Do the best you can to control the environment of the sugar glass.