Winston decides to rent this room because he wants a place to stay where he will be out from under the eyes of the Party. He thinks the room over the shop will give him such a place.
He also feels nostalgic about the room. It reminds him of better times, times before things got to be the way they are now. He felt that it was the sort of room in which you could actually be alone with your thoughts.
So Winston takes the room because it reminds him of the past -- of a time when a person could actually be alone. This makes sense because what he is trying to do by rebelling is to go back to a time when people could think for themselves and be who they wanted to be.
Thankyou, that helps now when i realised that Winston actually hired the room above of Mr Charrington's shop.