24 Gb installed only 16 GB usable

You could maybe try to re seat each of them and make sure they are in properly, take 8gb out, reboot it then shut it down and put the sticks back in and then boot it up again.

This is a duplicate post.

Are you SURE you have "Windows 7 ultimate 64 bit" and not Home Premium?

Problem solved
I took ABEKL advice and flash the Bios to version F7. I was a bit reluctant to update to the new Bios via the Internet but in the end I bit the bullet and did it anyway, actually it is quick and painless, lots better than the old "floppy way"

Once the computer reboots with new BIOS 24 GB is instantly recognized, Windows has no problem using it. Yes it is W7 64 bits Ultimate.

Thank you everyone for your very quick response, your helps are greatly appreciated

Mais51

Yeah Duplicate Post