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When you boot your computer there is a small utility called setup, which resides in the operation system and is responsible for setting up your devices and making the system ready to use. This setup program loads or installs the device drivers depending upon the need. For example in the initial stage of booting the setup loads only the Keyboard, Mouse, Video adapter, SCSI/Disk, and Machine/HAL. These drivers are very much necessary for the setup to continue booting of the computer. This stage is mainly referred as text-mode setup.
Once the text-mode setup is completed, the setup program then boots the operating system and goes on with the GUI (Graphic User Interface) stage of booting. During this stage most of the devices are installed in the computer.
Few devices may not get installed during this stage because may be they are being installed for the first time and needs users interaction for configuration or the setup was not able to find the appropriate device drivers to be loaded.
As soon as the computer is booted up and is in a running state, you can setup the required devices using Add/Remove Hardware wizard to install these devices. 
