Exactly, nowadays windows defender is much more competent than it used to be.

I think that your brother is thinking of liquid crystals, which are harmless.

You don’t need a whole new boot drive, just make sure it is completely formatted and reinstall. Just delete all partitions in the windows installer and install to the unallocated space and you will be good.

If you want it to last, buy/build another PC. Or even just upgrade your current one. An ssd can do wonders for an old computer.