The fact that this can't be done by default drives me nuts (that goes for KDE too). This quick article tells about an application that allows it, along with rotation of your systray icons as well.
http://www.howtogeek.com/howto/wind...ur-taskbar-buttons-and-tray-icons-in-xpvista/
http://www.howtogeek.com/howto/wind...ur-taskbar-buttons-and-tray-icons-in-xpvista/