When you move the mouse over the Windows XP taskbar icons, some tooltips are hidden by the taskbar.
This behavior will occur if you right-click in the Start menu's My Documents or My Recent Documents folder and Open a file.
NOTE: Microsoft is aware of this issue.
To workaround this behavior, log off and log on.
NOTE: For an additional workaround, see More on tooltips for some taskbar icons are hidden behind the taskbar on Windows XP?