What is a good application that will let me use my computer's keyboard like a synthesizer or a drum machine?

I’ve done the google already and found nothing. I’ve found some online Flash applications but the latency is awful. I found one tool on a tinyapps website but it’s for midi audio, i’m looking for something, preferable freeware, that can give me good quality synth control or an ability to map my own wavs to certain keys. I’ve found similar programs for linux but i’m looking for something to use with WinXP.