The following text has been taken from http://support.microsoft.com/kb/307729.  Please click this link for the full Microsoft article.

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
322756  (http://support.microsoft.com/kb/322756/ ) How to back up and restore the registry in Windows

To disable balloon tips in the notification area, follow these steps:

• Click Start, click Run, type regedit, and then press ENTER.
• Locate the following subkey:
  HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced
• Right-click the right side pane, create a new DWORD value, and then name it EnableBalloonTips.
• Double-click EnableBalloonTips, and then give it a value of 0.
• Close Registry Editor.
• Log off Windows, and then log back on.