particle help!!!

I want to make some particles disappear at a certain point (or keyframe) in blender but the layer key seems to have been removed in blender’s newer versions. I tried switching off the visibility icon in the outliner panel but the meshed seem to appear in the final render. Also at the end of the animation I want the morphed mesh(from a text converted into a mesh) to dissolve. I tried to use brownian motion animation but the particles seem to cage and not disperse… Please help me out with this. I am currently using 2.70a version of blender.

I am currently using 2.70a version of blender.
Why ?

Rather than us having to try and decipher what you are talking about please supply an example blend file and some annotated screenshots showing precisely what you have, what you want and the problem you are seeing