What is cumbersome on installing gimp? Just go to
http://gimp-win.sourceforge.net/stable.html and install the two packages. Hardly any more bothersome than any other windows programs (linux people have an easier time, but that applies to all free or open software)
Using GIMP is another matter, the interface is inconventional to some degree. Still, it is not exactly hard, and tutorials abound. I know of no other paint program that span both linux and win32... though on Linux there are alternatives such as Krita.