Normally, all you have to do is position the cursor (arrow) at the top of the task bar and then left click and "drag" the bar down and it will collapse back to one line.  Once it you get it on one line, RIGHT click in an empty area in the task bar and then left click on "Lock the Taskbar".  That will prevent it from accidentally moving or resizing.

I just read your post on the Windows XP software section, which states something different than what you posted here? 

 

What is the real problem?  And the Windows XP Software section is the most appropriate forum section for whatever your problem is as it is software and Operating System related.  It is not a "hardware problem" (this forum section).

I figured out how to change it into 1 line BUT before that the Taskbar moved

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