Fresh is always best,but frozen,tinned and dried are all perfectly fine. In fact-frozen is better in some cases,as the freezing is done before loss of nutrients.
You need to take into account not just the vitamins and other nutrients in fruit and veg,but also the roll they play in keeping your body functioning at it's best. A good diet is one with variety-not just rolls,chocolate (yummy as it is),and coffee....along with your main meal. Try a bit of fruit on your cereal, and remember that one glass of juice also goes towards your 5-a-day. It's not really all that difficult to do.