Protecting against bitcoin thieves as a casual user I hope this is an appropriate question here. I never got a chance to write it out earlier, but with the recent price changes looming it came to my mind again.
Around July 2013 I started to take some limited interest in bitcoin. Around this time, GPU based miners were still relatively viable, so I took my medium-grade gaming PC, set it up with a better GPU, and did a stint of casual mining. After some months, I had received about 0.7 BTC, but I generally deemed it not super profitable, and stopped. The PC was windows 7, used for the mining and a few games, and other general consumer tasks.
A few years later I became interested in the crypto space again and opened my old bitcoin client, only to find out that about a year after mining the coins (May 2014), they had all been transferred out somewhere unknown to me. At the time, for me, this pretty much destroyed most of the credibility of bitcoin, as from the perspective of a casual user, I had not really done anything wrong or unusually shady or dangerous.
I'm looking back into this space yet again after some of the recent news. So my question breaks down to:

What is the most likely situation that had played out, in order for me to lose my coins like this, on a casually used system, using the official bitcoin client?
If the above issue was due to some kind of known vulnerability, has it since been fixed?
What other measures can a new, non-power user take in order to protect from this type of significant theft, within reason?

Thanks for reading~
What other measures can a new, non-power user take in order to protect from this type of significant theft, within reason?

For money stored long term, move the coins to a hardware wallet or some form of offline wallet (cold storage).
Use a transaction (rather than importing keys or using an existing seed-phrase) to move the money to a wallet with fresh keys that have never been on any network-connected device.
Theft is not the only risk. Make backups of the wallet and/or seed phrase or keys. Test recovery from those backups. Educate heirs about recovery.
See What kinds of wallets are there?