It's an old one, but still one of my favourites:
Customer: I have just brought a new computer and I can't get it too work.
Techie: OK sir, can you tell me if there is are any lights on on the computer or the monitor.
Customer: No there are not
Techie: Well could you press the button on the monitor at the bottom on the left, do you get a light?
Customer: I have pressed it and no light.
Techie: OK we need to check the plugs on the back of the monitor, are they all fully inserted.
Customer: Yes.
Techie: OK you are going to need to check where they plug in.
Customer: I can't do that, it is under my desk and to dark to see.
Techie: Do you have a lamp you could use.
Customer: That would be no help you see we are having a power cut.
Techie: Well what you need to do is put the computer back in its box and take it back to the shop and tell them that you have to return it.
Customer: And why shall I say I am returning it.
Techie: Tell them that you are to stupid to own a computer!
And a few more that have sprung to mind:
Customer: I am having problems getting my new printer to work.
Techie: What is happening?
Customer: When I try to use the printer the computer says "I cannot see that printer" even when I put the printer right in front of the monitor!
Customer: I am having problems installing your software.
Techie: What is the problem?
Customer: I started the installation and all was going well until it asked me to insert (floppy) disk #2. So I did that and then it stopped
Techie: OK, try removing and reinserting disk #2.
Customer: It won't come out.
Techie: You need to press the button on the front of the disk drive.
Customer: That's not helping. I can't say I am surprise the amount of force I needed to get it in.
Techie: What do you mean?
Customer: Well with the first disk in there, it was extremely tight trying to fit the second disk in as well!
On-site Techie: I am sorry to say your data is corrupt. We are going to need to use your backup disks to restore the software. You do have backup disks?
Customer: Oh yes.
Customer then hands techie a ring folder with all the disks in. The disks have holes punched in them
On-site Techie: I am sorry to say the disk is corrupt. Do you have a copy of the disk.
Customer: Oh yes.
Customer then hands techie a piece of paper with a photocopy of the disks on.
Techie: I think I know how your disks became corrupted!
Customer: I won this Mac in a raffle and though I usually use PCs I thought I would try it. On looking at the spec I felt that it needed a new graphics card so I purchased and fitted one. Now my Mac does not work.
Techie: OK Sir, can you talk me through what you did?
Customer: Oh yes, well the case is not like my PC and I could not work out how to open it. So I got a hacksaw and cut the back of the case off...
Techie hangs up phone.