One of the first interview questions. You need to convince the employer that this job is not your last option. To say you want to have it because it was your field of study is like telling you must get it, not that you really want to have it. It should never happen to you in an interview!
Oppositely, you should try to show your motivation and enthusiasm for the job, showing some passion for desktop support and helping the others. Alternatively, you can say that you fulfill all the job requirements and believe to bring some added value to the company. Recruiters love such an attitude!
I always loved to troubleshoot computers. I am taking care of big LAN in my block. I read a lot about DS too and as soon as a new software is out, I am searching for bugs. What more, I really like to talk to people and this job is a unique combination of work with computers and people. I can imagine myself being really happy with this occupation.
I fulfill all the job requirements, as outlined in the job description. I believe to be a good match for this position and bring some added value to the company. Most importantly, this is a kind of work I like to do and therefor the motivation will always be there.