Programs not appearing on Taskbar (Win XP)

This is my mother's computer, so I'm not sure what she did there. She's not sure either, she just claims she's done nothing. All I can see is that nothing appears on the middle section of the Taskbar when I start up a program. I tried Chrome, Word, Notepad and My Computer. Everything works fine except that the Taskbar doesn't display the program tabs. The Taskbar looks otherwise normal: Quicklaunch and systray icons are present and functional. Alt+TAB works fine.

check she hasnt accidentally shrunk the program area of the taskbar to nothing (theres drag handles when 'lock taskbar' is turned off). If that doesn't help try running sfc /scannow