Windows XP taskbar is stuck on "always on top of other windows"

Windows XP taskbar is stuck on "always on top of other windows"

My Windows XP taskbar is stuck on "always on top of other windows". It's always visible, even when a full-screen app is running. I've gone into the properties of the taskbar and un-checked the box for "Keep the taskbard on top of other windows" but it makes no difference. checked or un-checked, it's always on top of other windows. Any ideas?