Nightmare

The worst thing to happen is when a customer has a problem with a computer but you can't replicate the problem. Worse in this case was that I sold the computer. The customer in this case had a computer that stalled in startup for 15 minutes and then stalled for another 15 minutes during Windows startup. Unfortunately, during visit number 1, the computer worked perfectly for me. I restarted 3 or 4 times and it started up faster than any of my machines.
Called back a week later, same problem. I came back with a spare keyboard as sometimes this causes a problem. This time at least the computer misbehaved for me. I swapped keyboards and it booted okay. Great, problem solved. I tried the old keyboard and the computer booted quickly. Mmmm, well maybe I cleared an issue in the Bios. I have seen this happen a few times with USB keyboards.
Another call a week later. Same problem. I had enough at this stage and was going to change the motherboard. I took it back to the workshop with the keyboard and the mouse and continually rebooted the machine sticking various things into the USB ports. I could get it to fail. Rather than put in a new motherboard, I reflashed the BIOS and updated all of the drivers that I could find and returned to the customers house. We then set to see what device would cause it to fail. I suspected the camera connection. However, when rebooting I noticed there was some delay. Even though it was switched off the only thing plugged in besides the keyboard/mouse was the printer. I pulled out the printer and the system immediately booted. I plugged the printer cable into the computer and XP froze. The problem was a short in the USB cable. A very frustrating time and shows that no matter how many times you seeing a particular symptom, the computer can throw up a new cause.