Turn off the computer.
Disconnect the AC power.
Remove the side-panel, to expose the motherboard.
Look to see if there is more than one stick of RAM.
If there is, remove one stick, noting from where it was removed.
Reconnect the AC power.
Turn on the computer, watching for the message "press the <blah> key to run HP Diagnostics".
Press the <blah> key.
Select the RAM test, and run it.
Did it "pass" the test?
If so, you now know that have removed the "bad" stick -- take it to any computer store, to buy a replacement.
If not, and there is only one stick left on the motherboard, that stick is "bad", and must be replaced.
Turn the computer off, by disconnecting the AC power.
