Nermin,
After the very first update of my FC12_64 I've got the same problem on a couple of Z800, and seems related to the video hardware.
My Z800s has 3 disks, 1 for the boot and 2  1 TB in software raid1 and a runnlevel 3, so I can log in normally.
A startx does a 2 seconds of heavy work on the disks then reboots and no more disks  are visible, so a poweroff is the only thing I can do.
However, using the kernel found in the installation dvd my Z800s runs fine.
The nomodeset solution proposed in the fedora12 bug lists and the bios update from HP doesn't solved the problem for me.
I'm still looking for a solution, even if the systems are running fine.
If you are still interested, I will file you the detailed description of my Z800s configuration  ( tomorrow I will be in office )
 
Filippo
 
 
Skype shinymetal-skype
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