My pc is recently bought, I don't think Dell recycles old video cards. Anyway the video card is a Geforce GT640.

I've recovered Win8 by using boot-repair / unchek reinstall Grub / keep active only "restore mbr". Now as expected, it is Ubuntu that is not avalaible. On the basis of my knowledge, it is better for me to avoid other attempts to install grub. I think I've rejoiced as I've seen the rolling Windows progress appair under the Dell logo.
I think I'll wait for ubuntu precise pangolin LTD 12.04.2 last news say coming on February. Bios/uefi, bootloader are not toys. Or better, they are not toys for me.