Right now, my daughter's laptop has a periodically recurring problem with the LHS Toolbar - it keeps shrinking in width and/or refusing to "auto-hide", and I have shown her how to fix it with a restart of Windows Explorer. So far, I have not been able to establish what is causing this recurring problem.
Not running a game, (or other program), that causes the Windows Aero features to be turned off, (ie. reverts to basic scheme) ?
The original Ghost Recon is an example of where that will happen.
Also, from here
Having the exact same problem. Status bar on left and autohide is on. When coming out of hibernation, connecting external display, or doing other screen related things, the width setting is completely forgotten.
And that's from 2010.
Being a laptop, do she by any chance let it go into Hibernation ?
Although, I've never seen it do it on my XP netbook, (LHS AutoHide Taskbar), and it normally only gets hibernated.