You don't seem to understand Krugman, or perhaps it is the broken window fallacy itself. Krugman is not advocating redirecting spending from one use to another (i.e. buying glass instead of buying bread).
Krugman is advocating increasing budget deficits and printing more money to increase aggregate demand. There is a very obvious increase in the amount of money in circulation in the economy in this case.
Do you know how "printing money" works in the US? And why it doesn't create much inflation? It is impossible to create new money for additional spending from thin air; but it seems that it is possible to bring them from future, specifically, from future taxes (since the government doesn't produce much).