How is Winston Smith upsetting or reinforcing the Party?
While Winston himself is not an important Party figure, his job is. He works at the Ministry of Truth, or Minitrue, where he "rectifies" old news articles and other information to bring it in line with Party orthodoxy. In the world Orwell has created, control of information is essential to maintaining power, and so Winston is very much reinforcing the Party's position. As perhaps the most famous quote from 1984 says:
He who controls the past controls the future. He who controls the present controls the past.
And of course, by the end, Winston has had any opposition to the Party purged from his mind by the brutality at Miniluv.
But Winston has long held a deep distaste for the Party, and struggles, mostly internally, throughout. He also dares to share a brief moment of humanity with Julia. But the struggle is, of course, completely futile, and by the profoundly pessimistic end of the book he is completely broken, both physically and mentally. The Party even controls his memory. We see by the end of the book that working against the Party in the open, as Winston did by embracing O'Brien as a potential ally and leader of the Brotherhood, only strenghthens its grip on individuals.