Ok, I answer to my question :-D
I have finally found a solution.
In fact, I have uninstalled all the fonts packages, and make some cleaning in the fonts directories.
After that, I have installed one by one the fonts packages, and relaunch KDE all time.
At a moment, I succedeed to have a clean font in kde ( without antialiasing ), with Helvetica, and 12 for the size.
After, I have installed the "urw-fonts-2.0-11mdk", relaunched KDE, and the fonts becam ugly ( alway Helvetica size 12 )
I have desinstalled this package, relaunched KDE, and the fonts became clean ! ( alway Helvetica size 12 )
This package is used by ghostscript and xdpf, and provide this fonts :
/usr/share/fonts/default/Type1
Somebody know how to not use this font by default in KDE ?
I have tried to change the line order of the Section "Files" in XF86Config-4, if there is prority in fonts using, but it's not working
A solution is to not install this package but if I open a pdf document with xpf or ghostscript, the document is all blank ! :-D
I have also solved another problem : to have same fonts display with non kde application.
Edit the ~/.gtkrc-2.0 file and put the line :
gtk-font-name = "Helvetica 12"
Regards