[Edited 2011-03-31 to fix a typo. Also note that WIN+T in Windows Vista/Windows 7 will set focus to the first taskbar item]
[Edited 2004-01-29 to fix mistakes]
Windows 95 introduced the “taskbar.” Designed to remind users what programs were running and to provide a single STARTing point, it was intuitive and original1.
I didn't join the Windows team until late in the development cycle and there was no keyboard access to the taskbar, often known as the “tray“. With Nashville, the codename for shell-intergrated Internet Explorer, the taskbar code was completely rewritten and given basic keyboard access.
The taskbar is really a series of sections. At a minimum, there will be three sections. The first is the Start button, the second is the series of open windows commonly known as the taskbar. The next section will be the taskbar notification area, which includes the clock and any notification icons, like the Volume control, and Dial-Up networking status.
Normally, the taskbar doesn't have focus - one of the open windows does. There is no direct keyboard shortcut to the taskbar, but there are a couple of ways to get it focus. The first is to open the Start menu by pressing it's shortcut. That would be the Windows key, or CTRL+ESC for keyboards without a Windows key. The Start menu will appear and have keyboard focus. You can use the arrow keys to move around the menu and press Enter to select an item.
To set focus to the taskbar, you can press ESC to dismiss the Start Menu. The keyboard focus will then be placed on Start button. Notice the faint dotted rectangle indicating focus. Pressing Space or Enter will open the Start Menu again. Pressing the Context Menu key or the older SHIFT+F10 shortcut will present the shortcut menu for the Start Menu.
With the focus on either the Start Menu itself, or the Start button, pressing the TAB key will move focus to the next section. This maybe the Quick Launch toolbar, or the buttons representing open windows. Each acts like a toolbar, and you can move between the items with arrow keys. Pressing Space or Enter activates the item just as if you clicked on it. Similarly, pressing the Context Menu key or SHIFT+F10 will present the menu for that item.
Pressing TAB again moves to the next section - either an enabled toolbar or if none, the tray notification area. The focus will be placed on the first icon. Unlike a toolbar however, the tray icons have three methods of input - primary (left) click, secondary (right) click, and primary double-click. When keyboard focused, pressing Space performs the same action as primary click, while the Enter key is the same as a primary double-click. The Context Menu key or SHIFT+F10 would do the same as a secondary click, normally presenting a context menu for the item.
To see this in action, play around with the Volume icon. It's disabled by default in Windows XP, you might have to enable it first.
A single click on the Volume icon presents the volume slider control. When focused, the slider will appear after pressing Space. A secondary (right) click produces a context menu with two items, “Open Volume Control“ in bold letters and “Adjust Audio Properties“. The bolded item indicates the default action when the icon is double-clicked. To replicate the double-click, press Enter and the full Volume Control appears with several sliders.
The notification icons are always the last section. If you press TAB while on any icon, focus is given to the desktop. Note that focus is actually on the Desktop itself since no icons are selected. Pressing Context Menu or SHIFT+F10 here gives you the (in)famous desktop menu. Pressing Space will select whatever icon has focus.
Pressing TAB while focus is on the desktop will move focus to the next toolbar is enabled (such as Address bar docked to one edge of the screen). If no other toolbars are enabled, focus will be given to the Start button and the cycle is started over.
Basically, if you are a keyboard-only user, the taskbar has full functionality available.
There is one piece of functionality I didn't demonstrate, and that's the ability to turn on or off other Toolbars. That can ONLY be done from the taskbar context menu. Usually the cursor is placed over a empty area of the taskbar and a secondary click is done. A Toolbars sub-menu lists those installed. To get to the taskbar context menu, you have to get the focus on the clock and press the Context Menu key.
If you want to play around with the keyboard, note that the taskbar can be moved and resized by the keyboard. Press ALT+Space to activate the system menu. Choose Move and press the arrow keys towards the new screen edge the taskbar should dock to. Nothing will appear to happen at first, but as you get near the screen edge, the taskbar will snap from it's old location to the new one. Press Enter when finished to exit the Move or Size operation.
The keyboard access to the taskbar and desktop is hardly optimal, but fits with Windows overall keyboarding scheme. When other toolbars are enabled, quirks get into the mix, so I can't recommend them. For those users who are forced into keyboarding only due to physical or machine limitations, this functionality can be very useful.
1 No, the Apple menu bar does not count. Ever try figuring out what other programs are running on a Mac? Maybe it's easier now with OS-X.