Dell Vostro 220 (Late 2008) - Manual (Page 49)

Computer Cover
49
4 Holding the cover at a slight angle, slide it toward the front of the
computer until the metal clips inside the cover grip and attach securely to
the chassis frame beside the front bezel.
5 Ensure that the cover is seated and aligned correctly.
6 Replace and tighten the two thumbscrews that secure the computer cover.
7 Follow the procedure in "After Working on Your Computer" on page 45.