Making a copy of the hard drive should be the first action to protect the chain of evidence when a desktop computer is involved. A chain of evidence, also known as a chain of custody, is a process that documents and preserves the integrity and authenticity of the evidence collected from a crime scene, such as a desktop computer. A chain of evidence should include information such as:

The identity and role of the person who collected, handled, or transferred the evidence

The date and time of the collection, handling, or transfer of the evidence

The location and condition of the evidence

The method and tool used to collect, handle, or transfer the evidence

The signature or seal of the person who collected, handled, or transferred the evidence

Making a copy of the hard drive should be the first action to protect the chain of evidence when a desktop computer is involved, because it can ensure that the original hard drive is not altered, damaged, or destroyed during the forensic analysis, and that the copy can be used as a reliable and admissible source of evidence. Making a copy of the hard drive should also involve using a write blocker, which is a device or a software that prevents any modification or deletion of the data on the hard drive, and generating a hash value, which is a unique and fixed identifier that can verify the integrity and consistency of the data on the hard drive.

The other options are not the first actions to protect the chain of evidence when a desktop computer is involved, but rather actions that should be done after or along with making a copy of the hard drive. Taking the computer to a forensic lab is an action that should be done after making a copy of the hard drive, because it can ensure that the computer is transported and stored in a secure and controlled environment, and that the forensic analysis is conducted by qualified and authorized personnel. Starting documenting is an action that should be done along with making a copy of the hard drive, because it can ensure that the chain of evidence is maintained and recorded throughout the forensic process, and that the evidence can be traced and verified. Turning off the computer is an action that should be done after making a copy of the hard drive, because it can ensure that the computer is powered down and disconnected from any network or device, and that the computer is protected from any further damage or tampering.