I would go with a slightly water-damp cotton cloth for the outside. Any kind of Windex or cleaner is likely to start removing paint. Same for the keyboard, but VERY little water -- not even enough to squeeze a drop from the cloth.
For the screen, if a damp cloth doesn't do it, then I would suggest a very, very diluted solution of Windex in water, in very sparing amounts.
I wouldn't suggest using anything as harsh as alcohol anywhere on the PowerBook, but I'm sure others have with varying degrees of success.
Can't go wrong with water diluted in water.