Resolving "NTLDR missing" and "Non-system disk or disk error" error messages

I thought that this solution was common knowledge all around, but it seems not to be, so here’s how. First off, the problem. When you boot your computer, an error message pops up, saying one out of two:

NTLDR is missing
 
Press any key to restart

Non-system disk or disk error
 
Replace and strike any key when ready

The problem is one out of the following four (the third one is more or less an anachronism these days, but I’ll include it even so:
 

1. A USB drive is connected to the computer
2. A CD or DVD is located in the optical drive
3. A floppy disk is located in the floppy disk drive
4. The hard drive, or operating system in it, is faulty

 
The first three are simple enough to resolve; you simply remove the USB drive, CD, DVD or floppy disk, and reboot the computer. The fourth one is resolved by ascertaining whether the hard drive is dead, replacing it if it is, and reinstalling the OS.