from:
http://www.timeatlas.com/mos/5_Minute_Tips/General/How_to_Resize_or_Move_Windows_Taskbar/
Repositioning the Windows Taskbar
Another frustrating item about the taskbar is when it moves to some place you didn’t expect. This undocumented phenomenon tends to happen when friends or relatives have borrowed your computer. As example, instead of being at the bottom of your screen, your taskbar is now on the left side. You mistakenly think you can grab hold of the task bar and reposition it like any other window. You can, but there are two little tricks.
How to move the Windows taskbar,
1. Right-click on a blank area in the taskbar and deselect Lock the Toolbar. A divider line should appear next to the Start button.
2. Left-click the area between the Start button and your divider.
3. While keeping the left mouse button pressed, drag the taskbar to the new location. (top, bottom, left or right)
4. Release your mouse.
5. Right-click any blank area and Lock the Toolbar.
Although we’d like to say these problems only occur when small children are around, we know better. Some of you may have noticed that you didn’t need to unlock the toolbar when following the instructions. Yes, keeping your Windows taskbar unlocked does increase the odds of it moving around. As a big kid, I can attest to this.