The Day The System Tray Became the "Notification Area"
Controlling Windows XP's "Notification Area" (commonly called the "System Tray")
Windows XP Home and Professional

Most of you, in fact all of you, have icons in your Windows XP system tray (which is now called the "Notification Area" - the icon display area by the system clock). Sometimes this area can be a bit of a problem especially if you have a lot of programs running which display icons in this area. Most, but not all, of the programs that use the system tray are programs that start with Windows. When this area becomes crowded, Windows XP hides inactive icons. So if you haven't accessed a program for awhile Windows may hide the tray icon. But, in Windows XP, you can control the behavior of icons in the system tray. Here's how.

Right-click on an empty area of your taskbar, select "Properties". At the bottom of this dialog window you'll see "Notification Area" (System Tray). If you uncheck "Hide inactive icons" all of the icons currently displayed in the tray will become visible. If you leave it checked you can customize the "Notification Area" by clicking "Customize". You can choose from the list which icons you want to "Always hide",  "Always show" or "Hide when inactive". If you have a program who's icon you'd like to see always show in the system tray, which always seems to be disappearing, you know how to fix it now, don't you? And if you have an icon that you never use and is taking up precious real estate in your "Notification Area", you know how to fix that too!