Machinima for movie contest

[Delirious]

I am thinking of making a public service announcement on drunk driving for a nation-wide contest involving prize-money. I would like to make a machinima(a movie created within a game)utilizing a commercial engine such as half-life 2 source engine. Is it legal to enter into this sort of contest with a machinima project?

[Yorick]

Hrm. It might be illegal on account of the prize money.

[Angry Lawyer]

Ask Valve, they'll give you the right answer.

-Angry Lawyer