This article presents the participation of the Intelligent Systems Group (GSI) at Universidad Polit´ecnica de Madrid (UPM) in the Sentiment Analysis workshop focused in Spanish tweets, TASS2015. This year two challenges have been proposed, which we have addressed with the design and development of a modular system that is adaptable to different contexts. This system employs Natural Language Processing (NLP) and machine-learning technologies, relying also in previously developed technologies in our research group. In particular, we have used a wide number of features and polarity lexicons for sentiment detection. With regards to aspect detection, we have relied on a graph-based algorithm. Once the challenge has come to an end, the experimental results are promising.