This happens when the OS is not installed on the drive that you're booting from.
Or when you have inserted a non bootable disc into an optical drive or a USB and the computer boots from it.
To rectify, you need to get into the BIOS and change the Boot Sequence, making the drive that has the OS installed on it the first Boot device.
This can also happen if a few systems files were accidentally deleted by a user from the C drive.
If the OS has not yet been installed onto the rig, then you need to first get a Bootable OS disc.
Startup the rig and get into the BIOS, by pressing Delete at Post.
Change Boot sequence to ODD first and then the Primary HDD.
Insert OS CD/DVD into the Optical drive and save and exit the BIOS.
Wait for the rig to boot from the CD/DVD and follow the procedure onscreen.