Performing a bit-by-bit backup of the hard disk using a write-blocking device is the first step to do when a forensic examination of a PC is required, but the PC has been switched off because it helps to create a forensically sound copy of the original evidence without altering or damaging it. A bit-by-bit backup, also known as a physical or raw image, is a complete copy of every bit on the hard disk, including the unallocated or deleted data. A write-blocking device is a hardware or software tool that prevents any write operations to the hard disk, such as updating timestamps or changing file attributes. Performing a bit-by-bit backup of the hard disk using a write-blocking device ensures the integrity and authenticity of the evidence and allows the forensic analysis to be conducted on the duplicate image rather than the original source. Therefore, performing a bit-by-bit backup of the hard disk using a write-blocking device is the correct answer.