dust in a light effect

I am trying to create the effect of dust in a spot light. I want a very subtle effect.

Basically a very small particles moving very slowly through the light.

1. I want them to move very slowly. The best I can do is about the speed of snow falling. Is this as slow as it will go?

2. I would like them to move in random directions, but be contained to a cone shape (spotlight beam). Is this possible?