But I searched and I could not find one which specifically showed how you solve the problem of already having an Electrum wallet in an old PC and wanting to make sure your coins are only on your new PC and not accessibly anymore on the old PC.
...And I seem to need top post this once again. The coins are not in the wallet, they're "on the network".
So if you restore 100 wallets from the same seed, all will have access to the coins. That's why, if you plan to sell the old computer/HDD, even if at first you restore your wallet on the new device, it should be safer if you
send your coins to a
newly created wallet on new device (but not at the fees we have nowadays).
And if the amount of coins is significant and you are not a technical person, it may be safer to buy a
hardware wallet and send your coins to that. Wallets with big money, on computers accessing the internet, especially Windows computers, are not a great idea nowadays.