Gah! I hate it when I make a "trivial" mistake on what ends up being a simple problem if I had just looked at it in the right way.

Okay, here's a revised version of what I wrote this (Monday) morning... Okay, it's completely rewritten, hiding my shame!

When we do a Free Body Diagram on the box, we find that the magnitude of the normal force on the box is simply equal to the magnitude of the weight. So:

Work done by friction, then, is:

Kinetic friction is always defined to be in the direction opposite the displacement. So and are at 180 degrees to one another, sooooooooo:

You can plug in the numbers from here. And the "-" sign is very important as it implies energy is leaving the system, so you have to include it.

-Dan