You want to turn off balloon tips, which frequently don't give any useful information, especially for experienced users, and are more distracting then they are useful, particularly when they show up in the Notification Area.
Using the Registry
Using downloadable software
TweakUI, one of a suite of free, unsupported utilities from Microsoft called PowerToys, can hide balloon tips. Get it from http://www.microsoft.com/windowsxp/pro/downloads/powertoys.asp. After you install it and run it, go to the Taskbar section and uncheck Enable Balloon Tips. Click on OK, and the balloon tips will vanish after you reboot.
Although balloon tips can be distracting, they also serve a useful purpose, so make sure you want to turn them off before using this recipe. For example, when you hover your mouse over an item on the Taskbar showing an open window, the balloon tip will show you the filename or web location of the window.
MS KB 307729, "HOW TO: Disable Notification Area Balloon Tips in Windows XP"