Is it a good idea for a prospective MS student to mention in his SOP that he has good expertise on software like Microsoft office?
No, because it does not distinguish you from other applicants nor explain why you would a good fit in the program to which you are applying.  The expectation in 2015 is that everyone likely to be in graduate school, and even undergraduates, can use the standard desktop software with enough proficiency to get things done.  If specialized software, something like Mathematica or SPSS, is needed in your field, the assumption is that you either already know about it or you'll figure it out.

If you haven't already done so, have a look at this: (Master&#39;s Degree) What are they &#39;precisely&#39; looking for in a Statement of Purpose, and does a SOP matter when one has excellent GPA?