Particle Deflection or Collision

I am a 3D graphics artist for a living, and I am doing a project with a robot welder.

I am using the particle system in an animation. I want the particles to collide, or be deflected by an object. I am using 2.47, and I would like to know how to set an object to deflect particles.

Thank you.

Hello! Look the image below!


Thank you very much for your the help.

I also found out that the collision object must be on the same layer as the emitter.