The NA60 experiment studies dimuon production in p-A and In-In collisions with a much better quality than previous experiments. This is mainly due to the use of radiation-tolerant pixel detectors, placed immediately downstream of the target, which allow us to track the hundreds of charged particles produced in a high-energy nuclear collision and select, among them, those that match the two muons which gave the trigger. The matching procedure significantly improves the dimuon mass resolution and the signal to background ratio, while the measurement of the impact parameter of each muon track with respect to the collision vertex, thanks to very good vertexing capabilities, allows us to measure the open charm contribution to the dimuon spectrum, given the long lifetime of D mesons. In this talk, after explaining the data analysis steps (reconstruction, muon track matching, background subtraction and vertexing), we present the first quantitative results on open charm production at the SPS.