First of all, ActiveSkin (SkinBuilder) is not available any more... God knows why, but web site doesn't provide link for download anymore...
Second thing... (reply to mr.morden) that program is not standard basic skin maker as you may think - one of those "load picture, save skin and run it". This one is much more complex, plus, the program unfortunately is full of bugs.
Advice for mr.morden:
First step - make your preview of the future skin you want to build. Use Photoshop or any other good drawing utility.
If you see that picture and your idea, both looks good and it would be nice skin out of that idea, then start to think about learning how to do it in ActiveSkin, 'cause, believe me... there is lots of work... lots of...
You have to draw basic picture (background for the main pannel), every button state (that means, for each button you can have minimum 3 states - normal, mouse hover and pressed), you have to build transitions for every single button and you have to build and control animations. Last one isn't easy at all, especially 'cause ActiveSkin has ugly bug in itself and make your process of making animations, miserable (you'll pull your hair out of your head...

)
I'm not trying to discourage you, but I'm just saying that you must be more than ready to start with this... BTW, we hope that we'll see some more avast! skin designers in the future... anyway, we have to do something with this program (SkinBuilder), 'cause it's almost impossible to work in this conditions...
Cheers !