It's very easy.
Move the mouse cursor into the task bar area, left-click and hold it then drag it to the bottom. You're DONE!!!!!
Regards,
Hello:
Can anyone please let me know how to move my tak bar in Windows XP "where it belongs," at the bottom of my Desktop? (Somehow I moved it the the top, and it's very annoying.)
Please advise. Thank you.
-John

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic