Converting Mountain Bike for Road to complete in 33 mile mini triathalon
Even if you do not go as far as different rims, consider different tires.  

Slicks (1.25") vs the regular tires (2+ inches) will make a huge difference, and usually can be found for $30 a tire or so in the US or Canada.

Beyond that, remove unnecessary hardware.

Front shocks are probably the worst thing you might have on the bike.  I do not know if it is worth considering a hard fork just for this race or not.