This can happen if you have an incorrectly configured personal firewall, such as those from Symantec (Norton), ZoneAlarm or BlackIce Defender. These tend to default to a 56K dial up connection when they are installed.
If you then install your Tiscali broadband connection it can either slow you down to modem-like speeds or block your connection entirely.
If you have this problem, the simple answer is to uninstall your firewall completely. You need to check the help guide for your firewall for full instructions on how to do this.
Then reinstall the firewall from scratch, making sure that you select ADSL as the type of connection during the installation of the firewall.