Taskbar buttons moving from left to right on startup

Every time I restart my machine, my taskbar buttons 'magically' appear on the
right hand side of the taskbar. This machine only started doing this a
couple of weeks after I unpacked it. I know the fix by unlocking the taskbar
etc, but I want to know how to stop this from happening in the future (it's
an annoyance every time I want to restart the machine). Anyone out there
know how to help?