A good one!

A guy rings tech support to report that his computer is faulty.

Tech: "What's the problems?"
User: "There is smoke coming out of the power supply."
Tech: "You'll need a new power supply."
User: "No I don't! I just need to change the startup files."
Tech: "Sir, the power supply is faulty. You'll need to replace it."
User: "No way! Someone told me that I just needed to change the startup and it will fix the problem! All I need is for you to tell me the command."
10 minutes later, the User is still adamant that they are right. The Tech is frustrated and fed up.
Tech: "Sorry, Sir. We don't normally tell our customers this, but there is an undocumented DOS command that will fix the problem."
User: "I knew it!"
Tech: "Just add the line LOAD NOSMOKE.COM at the end of the CONFIG.SYS. Let me know how it goes."

10 minutes later.

User: "It didn't work. The power supply is still smoking."
Tech: "Well, what version of DOS are you using?"
User: "MS-DOS 6.22."
Tech: "That's your problem there. That version of DOS didn't come with NOSMOKE. Contact Microsoft and ask them for a patch that will give you the file. Let me know how it goes."

1 hour later.
User: "I need a new power supply."
Tech: "How did you come to that conclusion?"
User: "Well, I rang Microsoft and told him all about what you said, and he started asking questions about the make of power supply."
Tech: "Then what did he say?"
User: "He told me that my power supply isn't compatible with NOSMOKE."