I use a combo of Taskbar++ and TaskbarEx in XP. (not sure how these 2 apps behave under Vista)
Taskbar++ let's you hold down your alt key while hovering over the taskbar, and drag & drop to reorder the buttons in any order you wish. (page is in Japanese, just download TaskbarPP12.zip, which should be the first link on the page.)
You will need to turn off automatic grouping for similar buttons in your taskbar settings.
TaskbarEx lets you drag & drop the buttons off the taskbar to anywhere on your desktop and arrange them any way you wish. The buttons are skinable, so if there isn't one to match your colors or theme exactly, you can always make your own. Right click the buttons after dragging to the desktop to see what other settings are available, and for the option to return the buttons to the taskbar.
