For the word interest we can say:
I am disinterested* in that topic.
And it is correct. To be correct again we must use the prefix "un" if we choose to structure the sentence this way:
That topic is uninteresting.
Why is it like this? Is there perhaps an Old English rule that determines the negative prefixes that the common English speaker has since forgotten? Are there other examples? The Merriam dictionary linked to below says there is an argument over this particular word, so maybe this situation is unique to this word. Maybe the answer is in the exact purpose of "dis" and "un".
* It seems that uninterested is also a word but I never hear anyone use it. There is apparently an old argument over this too.
Best Answer
I thought I'd better offer an answer rather than a comment here, after doing a little research.
Traditionally, there was / is a famous distinction between uninterested (heading towards bored) and disinterested (impartial), but the distinction is apparently (and sadly) becoming blurred. Though many people would want to maintain the useful distinction (I'm one of these), English is ultimately a communication tool, and we must bear in mind the ways a substantial proportion of speakers use words. These change over time, and the process seems largely unavoidable, even in cases where people more interested in the language per se than the majority of speakers are consider the changes for the worse.
At http://grammar.about.com/od/alightersideofwriting/a/disuninterest.htm is a fine series of comments about the uninterested / disinterested debate, including discussions about how far the blurring has gone / 'is acceptable'. The AHDEL has a valuable usage note:
(The American Heritage Dictionary of the English Language, 4th ed., 2000)