I don't know what a public and/or private key is.... (as many others)
Which wallet do you mean? MXC1, MXC2 or Master. Or the MXC2 with the err extension?
Make a backup of each of those wallet.dats, now.
You can try them one by one by putting them in the bitcoin app data folder. Once Bitcoin Core starts it will work with whatever wallet.dat is present in that folder. Give it time to start and also to stop. Check in the task manager if you are unsure if it is still running.
Do you know which address holds the coins that you want to recover?
Step-by-step instructions for working on a single address here:
http://computriks.com/en/bitcoin-export-private-key
For working on a whole wallet.dat look into pywallet http://www.devtome.com/doku.php?id=pywallet
It is somewhat more complex though.
I installed Electrum. At startup it is asking me:
- Create new wallet
- Restore an existing wallet from its seed
- Create a watching-only version of an existing wallet
...what should I choose ?
I doesn't need to be Electrum. Once you have your keys and a properly working Bitcoin Core again, you can import private keys there.
http://computriks.com/en/bitcoin-import-private-key
As a side note: Having different wallets running on the same machine within VMs does not add any safety, IMHO.