Kriyas Megilah takes precedence over many mitzvos. This is due to the importance of publicizing the nes of the Purim story. Some exceptions include Kriyas Shema, tefilah, burying the deceased, and--according to some--bris milah. Even aside from these exceptions, if the opportunity to do a mitzvah arises and it will not be possible to do it after the Megilah reading, the mitzvah should be done first. If the performance of the mitzvah will stretch beyond the time frame of Kriyas Megilah, the poskim disagree about whether it would still override Kriyas Megilah or whether the Megilah should be read instead. Some say that in this case, reading the Megilah takes precedence, since publicizing the nes overrides the performance of other mitzvos.
[שו"ע תרפז, ב, ומשנ"ב ח-יב; ביאורים ומוספים דרשו, 15 ,17 ,ו-18]