This should be diagnosable by using the computer a bit and seeing where it struggles.
Your processor will need to be upgraded if your computer is having issues handling large calculations such as large table results from excel or other computational tasks.
Memory needs to be upgraded if you notice the Memory graphs are always maxed out in the Windows Task Manager ( ctrl + shift + esc).
Graphics would need to be updated if you're having issues with anything processing on the GPU, or windows is suggesting that you turn off certain features like Aero.
HDD will need to be upgraded if you spend too much time starting up your computer, explorer is taking too long to open a directory, or a file is taking vastly too long to open.
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