There is not getting around this question, the gap in your CV or what you have put on an application form has given them this information and you need to be honest about it.
Before the interview, make sure you have given this some real thought. Why were you unemployed for such a long time? Employers are fully aware of the economical climate and the high unemployment rates however what you will need to demonstrate it what have you been doing to find work?
If the answer is not a great deal then unfortunately you do not stand a great chance but if you have genuinely been looking for work then when you are asked this question you can answer it correctly by saying what newspapers you have looked in for jobs, what jobs you have applied for, what websites you have looked at, have you sent your CV speculatively to anyone, joined any recruitment agencies, the job centre etc?
If you can show your employer just exactly what you have been doing during this period of unemployment it will look much better because an employer will be worried that if you have a bad day at work you will leave them and no employer takes someone on thinking they will leave in under a year.
Once you have demonstrated what you have been doing to find employment you will most likely be asked, why do you think you have had no success in finding a job after all that you have said? You will also have to tell them if you have had any interviews, if unsuccessful why, if you think that you weren’t quite suited to that role and you will have to show that you are looking for a chance to prove yourself to an employer and are prepared to work as hard as possible to ensure that you learn everything there is to know and are truly a committed individual.
A difficult situation understandably just please think about your answer carefully before your interview to ensure you get given a good chance.