Dell Dimension 2100 System Reference Manual page 16

1. Remove the computer cover.
2. Lay the computer on its side.
3. Press the release latch while lifting the power supply.
4. Rotate the power supply out of the computer while keeping the drive power cables clear.
5. When you rotate the power supply back into the computer, gently lift the and hold the drive cables out of the way.
6. Rotate the power supply into position until its release latch clicks. Lay the drive power cables along the top of the latch.
Front Panel
1 Release latch
2 Power supply
3 Drive power cables
1 Drive power cables
2 Power supply