not GPU option in cycles

Hello, i have a nvidia 8600GT with the lastest drivers and this build of blender but not appear the option for use GPU rendering in cycles, what can be the problem?
cpu: intel q6600
ram: 4GB
OS: xp64 sp2
board: gigabyte ga-p35-dls3

I uninstall and reinstall drivers and use a last build for x64 and now work very fine