There are [at least] a couple of ways. The easiest might be to display the
document, press Alt+PrintScreen to copy it to the clipboard, paste it into a
graphics program, crop to just the portion you want, then save it as a .jpg,
etc. Then point the screen saver at that file for the image.
Another way would be to open the document in Word and select the part you
want to become a graphic, and copy it to the clipboard. Then, choose Edit -
Paste Special - choose one of the picture formats. Copy the resulting
picture to the clipboard, then paste it into a graphics program (such as
Windows Paint), and save it as a graphic that the screen saver can use.