put the new word in a movieclip (select and press F8), then click the movieclip and in the properties box, change the instance name to something like word1 and put this script around the part that makes it move

if(word1._x >= 50) {
THE SCRIPT THAT ALLOWS IT TO MOVE RIGHT
} else if(word1._x <= 100) {
THE SCRIPT THAT ALLOWS IT TO MOVE LEFT
}

if you dont understand that, post the script youve got and ill put it in for you where it needs to go, i dont know exactly what your script is like so i dont know exactly what i need to write
NOTE
Script above is AS2