Azu, yeah, Melty Blood is the only fighting game that you must have for PC. The full title is Melty Blood Act Cadenza Ver. B. It's an absolutely phenomenal game -- easily one of my favorite 2D fighters. It basically has the speed of Guilty Gear, but without some of the needless complexity (that's not to say there's no depth though - there is.) You can also use a 3rd party program to play online, though it's not really that convenient of a program, so expect to play on your own most of the time.
Apart from that, though, there's only Street Fighter IV on the horizon for the PC. Presumably Tatsunoko Vs. Capcom, as well, if Capcom keeps working the way they are.