Your initial and primary job is to preserve the computer evidence and to transport the computer to a safe location where a complete bit stream backup of all stored data areas can be made. You also want to insure that the computer system can be reconfigured to match the configuration in which it was found. For this purpose, it is wise to take pictures of the complete computer system from all angles. Wires should be marked such that they can be easily reconnected. Also, the computer should be clearly marked as evidence and stored out of reach of inquiring co-workers.
Chain of custody is as relevant when it comes to computers as any other form of evidence.
Law enforcement agencies have come under scrutiny in recent times regarding evidence issues. For this reason, it is important to do things right. Be sure to properly document the time, date and circumstances surrounding the actual seizure of the computer. This helps rebut the contention later on that the evidence on the computer was planted by the computer specialist. Every effort must be made to show that no one could have made changes to the information contained on a seized computer system.