Tower style computers have a monitor that is separate to the monitor. The tower would sit under a desk, by a desk or on top of the desk with the monitor attached by a cable but still being a separate entity to the hard drive.
video card
how many types hard disk drives
you can't buy it separately from gamestop but you can find it on eBay separate
Hard Drive..!
because if you hard drive or monitor crashes it will save on the hard disk.
Performance Monitor
ram
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If they are on separate hard drives you can generally just delete/format the hard drive with the OS that you do not wish to keep. If they are on separate partitions on the same hard drive you may have to reinstall the bootloader after deleting/formatting the partition.
the monitor, the hard drive and the key board/mouse
hibernation
no all information is stored on the hard drive of the coumpter .