"dis" and "un" prefixes for the word interest

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:

In traditional usage, disinterested can only mean 'having no stake in
an outcome,' as in Since the judge stands to profit from the sale of
the company, she cannot be considered a disinterested party in the
dispute. But despite critical disapproval, disinterested has come to
be widely used by many educated writers to mean 'uninterested' or
'having lost interest,' as in Since she discovered skiing, she is
disinterested in her schoolwork. Oddly enough, 'not interested' is the
oldest sense of the word, going back to the 17th century. This sense
became outmoded in the 18th century but underwent a revival in the
first quarter of the early 20th. Despite its resuscitation, this usage
is widely considered an error. In a 1988 survey, 89 percent of the
Usage Panel rejected the sentence His unwillingness to give five
minutes of his time proves that he is disinterested in finding a
solution to the problem. This is not a significantly different
proportion from the 93 percent who disapproved of the same usage in
1980."

(The American Heritage Dictionary of the English Language, 4th ed., 2000)