Functional fixedness for defining problems
What you are describing is called mental set. The following extract is from this excelent article.


  A mental set is a tendency to only see solutions that have worked in
  the past. This type of fixed thinking can make it difficult to come up
  with solutions and can impede the problem-solving process.
  
  For example, imagine that you are trying to solve a math problem in
  your algebra class. The problem seems similar to ones you have worked
  on previously, so you approach solving it in the same way. Because of
  your mental set, you are unable to see a simpler solution that might
  be possible.


Additional reference:

Galotti, Kathleen M. (5 February 2009). Cognitive Psychology: In and Out of the Laboratory. Cengage Learning. pp. 341–344. ISBN 978-0-17-644065-7. Retrieved 25 March 2011.