On the taskbar, I prefer keeping related program buttons close to each other. This saves me a bit time and hesitation on finding the program I need to switch to. While this appears supported in Windows 7, it is not in Windows XP.
Fortunately, someone created the lightweight program Taskbar Shuffle for XP, which lets you move the taskbar running program buttons freely around.
It appears to have a small bug: sometimes running programs disappear from the taskbar, although they are still running. Fortunately, they are still out there, and you can call them using Alt+Tab, which also forces them to reappear on the taskbar.