Thanks a lot Eddie, i owe you a dinner..

i tried to go in -s user mode but it didn't work. it stopped booting before loading the bash
i tried then to use the installation dvd terminal and to execute your instructions. I found three Extensions.mktext files that i deleted, and on the disk root directory many directories were marked with a @ right after the rw-r(etc) line.
However i booted and came back to "still waiting for root device" problem..
I'm thinking that maybe in my Acer Aspire 5101AWLMI the 10.5.2 can't be installed.. in your opinion may i have some chanches?