If you go to the Sagem website and download the latest driver for the Sagem 800 modem you can then run it from setup.exe.
This will first uninstall the drivers already (or presumably are still) on your system. You then run setup.exe again and, from the menu which appears, select the line "PPPoA vcmux 0,38". Continue.
The driver will now be installed. Reboot, reconnect your modem and Windows will find New Hardware; allow about half-a-minute for the modem to initialise and you should be back in action.
It is important to select the correct line from the menu; also check that 0,38 is listed as your telephone number in the Tiscali Broadband Connection configuration.