Is there any stand-alone software out there that can do realistic explosions or fire effects to be composited into an existing scene? I have an animation I did with an older version of 3dsmax (version 3), and need to composit some fx over the footage.

Attached Link: http://www.wondertouch.com/

I have to second that endorsement. ParticleIllusion is definitely what you are looking for here. Highly recommended.

Animation:Master has sprite particles built in that work very similar to particle illusion.

It's very easy to set up the particle emitters, they use .tga files with alpha channels as sprites, and they can collide with other particles, or anything else you can put in the scene with it.You can make your emitters whatever shape you like, in 3d, you can even set up invisible deflectors just like in particle illusion, except in A:M you can actually make the deflectors 3 dimensional.

A:M also has "Mouse motion capture", which means you can record while you drag the emitter around the screen, and play it back, just like in illusion. Or you can set up a path for the emitter to follow in 3 dimensions.

The only major differences are, you get less presets with A:M, and you save $100. ($200 if you get A:M at a trade show).