problem in spectrogram formation and segmentation

im making a senior design project on melodic pattern matching. im facing two problems. 1st in segmenting the spectrogram as it is too large and i need to convert it into a matrix on which i can apply algortihms. so help me in reducing the size of the spectrogram.(i have already used window and other fft functions bt it didnt help). my 2nd question: the spectrogram formation of a 3minutes song ususally hang my computer. what might be the reason behind it. please suggest the solution as soon as possible. its really urgent! please respond..