I hear "Kamusta" is now used today, "Kumusta" is old already...The only thing I hate bout Tagalog is that it changes overtime, out with the old in with the new. I am more confused than you are.

Both are correct

It comes from the Spanish greeting ¿Cómo está? so I guess Kumusta is the proper one, but Kamusta is widely used in the Greater Manila Area. Both are correct, it all depends on which region you come from. :)

Kumusta is the original form of this word. It becomes "kamusta" nowadays, just like Kamal have said, Tagalog language changes overtime. Just like "nakakapagtaka", it is originally "nakapagtataka", "makakapagtiis" from "makapagtitiis".. usually the word has the repeated part, and not the prefix.

Both are correct. *Changes in words used in languages are not that rare. If you actually study the English language, you'll find out that many words have changed their meanings over time.