If you're a demanding aesthete who insists on stamping your imprimatur all over your computer's interface, TweakXPlorer might be just what you've been looking for. If you're looking for a serious Explorer performance-enhancing drug, you're probably going to want to look elsewhere.
TweakXPlorer is a simple utility that lets you slap an image in Explorer's background, change font colors, and adjust background settings for the Explorer toolbar. Simple buttons with clean icons will help you navigate through the three main interface tabs.
There's TweakFolder for folder changes, TweakToolbar for toolbar adjustments, and TweakThumbnail to change thumbnail resolution and size. Importing an image into the toolbar automatically resizes it for the toolbar's rectangular dimensions, and the app makes use of sliders to avoid becoming visually stale.
It's portable, although the reasoning behind that decision is not exactly clear. Portable Firefox is something I understand, but for TweakXPlorer it just added several tedious steps before I could test it out.
You'll have to locate your images through the program's file browser, since drag and drop doesn't work here. Other than that, the biggest limitation the program faces is that it's kind of pointless beyond the skinning. If the visual customizations could be coupled with actual improvements for XP's Explorer, I'd be more willing to recommend the program enthusiastically.