Quote - an image is worth a thousand words.  How do you mean "dirty spots" ?
Hardware shading will only make any difference if your graphics card supports it I guess.  Mine has it set too for the preview but I don't think my card can use it

well I guess my do. Look over here; ati X1600pro 512mb. Have never looked up on that but that vga is apretty good one..I render a 300MB pz3 file in FFF in about 3 min time...all heavy lights,shadows etc..
but I'm here pal ready to learn..so...if you can tell me where do I disable that I will do ND TRY TO RENDER SAME IMAGE and compare.
here is the PREVIEW image.
http://www.renderosity.com/mod/gallery/index.php?image_id=1587812
 I'm rendering one with the dirty shadow in it..give me some time...

"'you shut up!  or I'll bring democracy to your country! "
Cris Galvão aka Softcris  - www.crisgalvao.com
(or softcris, SoftCris)
Rendering since 1997 and
at Renderosity since 1999.
OS Win 8.1     64 bit