What is Winston trying to learn in his conversation with the “old man”? Why? What is the result?

Winston wants to learn about the past. The totalitarian state has destroyed all independent historical sources; there is no longer any reliable means of finding out what life was really like before the Party took control. Winston spots an old man entering a dingy little pub. He figures that the old man must be about eighty years old and therefore must have been middle-aged when the revolution took place. He is one of the last surviving links with the capitalist era. Among other things, this means that his ideas were formed prior to the revolution; he could provide Winston with valuable information concerning the past. And as a prole, his truthfulness is not to be doubted.
Winston is driven by what he concedes is a "lunatic impulse," but he approaches the old man in the pub nonetheless. He wants to ask him all kinds of questions about life under capitalism:

Tell me about your life when you were a boy. What was it like in those days? Were things better than they are now, or were they worse?

The results of their conversation, however, are deeply disappointing. Winston and the old man are at cross-purposes. Winston asks a question about the most salient features of capitalism, but the old man can only seem to recall trivial details such as the top hats that people used to wear, instead of the economic system that they symbolize in Party propaganda.
Winston ruefully concludes that the old man's mind is nothing but a "rubbish-heap of details." He can't even answer the simple question whether or not life was better before the revolution. Winston is suitably deflated. It's not just the history books that have been falsified, not just official records, but memory itself. And when memory fails there's no alternative but to accept the Party's version of events as there's absolutely nothing it can be evaluated against in order to get at the truth.