Estimating a user's propensity to click on a display ad or purchase a particular item is a critical task in targeted advertising, a burgeoning online industry worth billions of dollars. Better and more accurate estimation methods result in improved online user experience, as only relevant and interesting ads are shown, and may also lead to large benefits for advertisers, as targeted users are more likely to click or make a purchase. In this paper we address this important problem, and propose an approach for improved estimation of ad click or conversion probability based on a sequence of user's online actions, modeled using Hidden Conditional Random Fields (HCRF) model. In addition, in order to address the sparsity issue at the input side of the HCRF model, we propose to learn distributed, low-dimensional representations of user actions through a directed skip-gram, a neural architecture suitable for sequential data. Experimental results on a real-world data set comprising thousands of user sessions collected at Yahoo servers clearly indicate the benefits and the potential of the proposed approach, which outperformed competing state-of-the-art algorithms and obtained significant improvements in terms of retrieval measures.