OK PROBLEM SOLVED!
I downloaded again the iso image this time using a ftp client and now the test cd utility of the linux installer said that everything was right. I still had the same problem: blank screen just after the messagges:
Running Anaconda, the Scientific Linux Sistem Installer - please wait
Probing for video card: nVidia Corporation Unknown device 0402
Attempting to start X Server
Waiting for X Server to start...
but this time I managed to install Scientific Linux (SL) v5 x86_64 64-bit version in textual mode
with no errors.
After the reboot, i downloaded the nVIDIA driver for Linux 64-bit with
and then i installed it with
Everything ok during install. I started the X Server with the command
and it correctly started! Then I also changed the line:
in the file:
to make 5 the default runlevel (multiuser graphic mode) so that X Server will start automatically at the end of the boot process.
Now everything is ok. Thanks everybody. Hope this could help somebody else.