Companies that rely on the Internet for their daily business are challenged by uncontrolled massive worm spreading and the lurking threat of large-scale distributed denial of service attacks. We present a new model and methodology, which allows a company to qualitatively and quantitatively estimate possible financial losses due to partial or complete interruption of Internet connectivity. Our systems engineering approach is based on an in-depth analysis of the Internet dependence of different types of enterprises and on interviews with Swiss telcos, backbone and Internet service providers. A discussion of sample scenarios illustrates the flexibility and applicability of our model.