This is a question I have been asked several times. It crops up so much I now include an explanation of the difference between the two words in our Sentence Works courses.
I think the confusion arises because the prefixes “dis” and “un” can both mean “not”. But disinterested and uninterested actually mean different things.
Disinterested means neutral or impartial. Uninterested means you are not interested in it, maybe because you find it boring. I never bother to watch the rugby results, for instance, because I am completely uninterested in rugby.
But if you were having relationship counselling, you’d go to see a disinterested party (rather than your partner’s best friend), because they are impartial. Other examples of people who are disinterested are juries, exam markers and umpires.
We also cover other pairs of easily confused words at our workshops. These include the difference between affect and effect, less and fewer, continuous and continual, and ie and eg. So do come along and find out the answers to these …and more!
For more examples, see http://www.english-grammar-lessons.co.uk/easily_confused/disinterested_uninterested.htm