If you're looking for a way to tweak Vista in just about every way you can imagine--and are willing to pay full freight to do so--check out Tweaking Toolbox Vista. With it, you'll be able to tweak just about every aspect of the interface, whether it be the Desktop, taskbar, Start menu, Internet Explorer, the Control Panel and plenty more.
The interface is straightforward, and although a bit unattractive, perfectly functional. Choose which part of Vista you want to tweak, and you'll see what you can change, with simple explanations for each. Then simply check or uncheck a box, and you're ready to go.
All that is to the good. But is the program worth $40? Unless you absolutely need to tweak many hidden settings, Tweaking Toolbox Vista is probably overkill. Many Vista settings are available via menus and options boxes, and so don't require a for-pay program for tweaking. But if you need to dig as deep as possible into Vista, and it's important that you do so, it might be worth the price to you.