Go to www.sospubs.co.uk an excellent resource for anything related to computer music making. There are so many options for you I won't repeat myself here - however a really great example is Reason - relatively cheap eminently powerful http://www.propellerheads.se/ and a great deal of fun although there are myriad others.
hmm, what type of music? Do you want to still make music using samples like ejaty? Or do you want to try midi based software? Or make your own synthersizor type music? There are also MOD sequencers called trackers though these take some time to get used to and are almost the same as midi software, though in my opinion, you have more control and aren't limited to just using samples on your sound cards library.