Adobe Photoshop
[link]Painter
[link] Artrage
[link]those are the most popular and professional for speed painting. (there are more though)
Gimp
[link] is also good AND FREE! So try that.
Do you have a tablet? if you don't, you will HAVE TO buy one. Painting with you mouse is close to impossible. The only good tablets are from Wacom
[link] look for the Wacom Bamboo if you are on a budget.
hope that helps!