My opiums.
I don't touch gimp it's insane.
Paint.net is pretty cool when you don't need all the hassle of Photoshop, and I can do art in it.
Photoshop is excellent for manipulation and art, but you'll have to go out of your way to get it, however you plan to.
Paint Tool SAI Is good for art but is shareware (you have to pay, has a trial though)
MS Paint, Vista, I used to art on this, it's not bad for quick cheap drawings.
MS Paint, 7, no.
Mypaint is amazing for art, look it up. It's free too.
Alchemy is great for generating ideas.