Dell Vostro 400 (Mid 2007) - Manual (Page 17)

Setting Up and Using Your Computer
17
13 power button,
power light Press the power button to turn on the computer.
The light in the center of this button indicates
power state. See "Controls and Lights" on page 172
for more information.
NOTICE:
To avoid losing data, do not use the
power button to turn off the computer. Instead,
perform an operating system shutdown.
14 drive activity light The drive activity light is on when the computer
reads data from or writes data to the hard drive.
The light might also be on when a device such as a
CD player is operating.