Computer can not do anything without a?

All the major componnets work together to make it operate. One could argue that a Power Supply Unit (PSU) would be needed first off in order to get the other components working.

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Power supply, hard drive, processor, system board, and RAM pretty much sums it up I think. All of these (or some substitution for such) are required. As long as you have these, it will work nailed to a 2x4 (yes, I have done it -lol-)

Case, fans, etc are nice but not required in the STRICTEST sense of the Word.