Napoleon's opposition to the windmill is simply explained: he knows that if the windmill is successful, and the animals have their workload decreased, they will be able to relax and think about how the pigs are exploiting them. Napoleon needed the animals to be overworked and exhausted so they will not question his authority; if they are able to have the easier life that was promised, he will be forced by public opinion to start working and stop taking more than his fair share from the public pot.
Napoleon, on the other hand, argued that the great need of the moment was to increase food production, and that if they wasted time on the windmill they would all starve to death.
(Orwell, Animal Farm, msxnet.org)
His direct argument against the windmill is rooted in a logical fallacy, the Appeal to Fear; if the animals want to eat, they need to follow his orders. When Snowball is expelled, Napoleon takes over the idea of the windmill as a propaganda tool and as another method to keep the animals overworked; they keep trying to build it, but keep failing, and as long as they are concentrating on the windmill, they are not thinking about Napoleon and his slow acquisition of complete power.