Typically, customers are non-technical people. That often results in frustration, as non-technical people tend to believe that technical people know everything there is to know and they are a specialist in every field. For them, if you can work on a website, you can also fix their MS Windows problem and so on.
This is not reality. So what do you do when the customer asks if you can look at their workstations too, with that eager look on his face ? Here's a few tips.

• First of all, keep calm. Let the customer finish his or her story first, that will keep peace.
• Explain clearly what it is you do. If you can, refer the customer to someone you can recommend to help out with the things you can't do.
• Setup a clear document of what services you provide, what your capabilities are and so on. Make sure each customer receives a copy of it.
• Take the customer by the hand and guide him or her throughout the process. That will make things clearer.
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  In general, keep calm, keep your respect for the customer (he or she is still the customer, paying your bill) and try and help out as much as you can, even if it is not in your field. If you have good connections, discuss with them and hire them if needed to get it fixed. It will make things easy for the customer not having to deal with several people.
  
  
  That's what big tech/IT companies do - they hire people in all fields.