Do the chkdsk/f and see if those errors go away, it will often fix issues, in admin command prompt put in chkdsk /f c: - it will likely need to reboot, and see

You can download memtest86 and use Rufus to put it on a usb drive and boot of that, if it do a full run through and if there’s and errors then your memory is bad

Screens and the tools are cheap on eBay and very easy to replace, do the screen yourself and upgrade the pc

Welcome to macs, everything is soldiered on and can’t be upgraded

Install a decent anti virus and check it, eset or bitdefender are decent ones, McAfee or windows defender are not

Use Rufus as the others have said, a file can’t be bigger than 4gb with fat32

Keep in mind the cpu will need power as well, if the video card needs 300w, go a at least 500w

Also have you plugged in both plugs? The main long one and the smaller cpu one?

I would not expect it to last long, liquid corrodes things extremely often in pc’s, I would make sure you get a backup of everything as quick as you can

You would have to find a battery that has a higher amount of amp hours and will still fit, it’s pretty unlikely, and a lot of those cheap ones are just glued together so not much chance of getting it apart without destroying it

Why would you not want to install an anti virus?

Unplug it, take out the screws and take the cover off, you should be able to remove any nails, and hopefully it still works Metal touching the motherboard is not a good thing

No windows will remember what letter it assigned to each