From what I've read, Gimp does indeed work, but the program hangs while opening fonts. You could also try pixia which was recommended in the art forum http://park18.wakwak.com/~pixia/index.html
Pixia is like a very old version of photoshop and isn't designed for image manipulation so much as coloration of manga.
(I don't know how to link things here yet)
Another more advanced program that has been used as a photoshop alternate is paint.net. Except for the gradiant tool and some of the plugins I'd say it's actually weaker than the Gimp, but it is said to be fully vista compatible.
http://www.getpaint.net/