go to Start>Control Panel>Internet Options
Click the connections tab along the top and highlight your tesco connection then click settings.
This will then open another window at the bottom of which you should be able to enter your username and password. Make sure you click apply before closing the windows.
It may also be worth calling tesco's customer services to check that the number being used to dial up is the correct number (unless you already know what number should be being used) as some viruses will change this and you can end up with some very large phone bills

HTH :)