A new trick, especially for pornography websites, is to install a program that changes your Modem dial-up settings to a premium-rate telephone number, so that you're charged an extortionate amount each time you connect to the internet. Premium-rate diallers are usually installed unwittingly. One false click can redirect you to an unsavoury website that appears with myriad pop-up windows.
Click on the OK button displayed in one without reading its message and you've just changed your dial-up settings to a premium-rate number - it's that easy. Besides steering clear of the sites that employ such underhand tactics, the best advice is to not to automatically click buttons in pop-up windows without first reading their contents. Failing that, you can usually uninstall any accidentally installed software using Add/Remove programs in Control Panel.
Some premium-rate diallers are written in a foreign language, which can make messages on installation or removal difficult to understand. If you are in this position then AltaVista's Babelfish service - named after the translating fish in Douglas Adams's Hitchhikers Guide
series of books - can do the translation for you. It's not perfect, but you'll get the gist of the message.