Open Registry Editor
Press Start and type “Registry Editor”. Click the top result.
In your Registry Editor search bar, paste the following:
Computer\HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\StuckRects3
In your main pane, double-click the Settings entry.
To enable auto-hide taskbar, find the row with the value data “00000008″. Double click the value in the first column, which should be “03”, and change it to “02”. When you’re done, press “OK”.