Did you check the hosts file mentioned on this page http://support.apple.com/kb/TS4451 ?
You can also try these instructions, some in-depth technical knowledge may be required here:
Also check that those ports are open, which are required by Apple products:
It is also possible that your Internet provider closes certain required ports.
Check with them, whether this is the cause of your problems.
Again, make sure that the firewall and anti-virus software are off for your installation.
Reactivate them after the installation!