Problem with volumetric light

Hello all
1- I used the volumetric light shader but the result wasn’t good
This is the result for VolumetricLighting

but it was good when use the bloom

what is the problem??
2- each time I use the the volumetriclighting it gives me an error

Cg program too complex for driver: created-shader. Try choosing a different profile.

I’m using GeForce8400
what is the problem

Thanks for that…I didn’t find it when searching on Google…
by the way the code was written

basic-shaders-only #t

so I just change the #t with #f