Typically, for something like a PC, there are few if any critical components. The whole PC costs less than $2000; what's critical is the information. That you simply backup. So if anything, I would vote for the hard drive in a critical application. (What is the application?)
There are a number of reliability standards that attempt to define critical components. They include operations and cost, in addition to safety. What is more important are critical failure modes; these you must design out or compensate for.
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