Detecting unknown worms is a challenging task. Extant solutions, such as anti-virus tools, rely mainly on prior explicit knowledge of specific worm signatures. As a result, after the appearance of a new worm on the Web there is a significant delay until all update carrying the worm's signature is distributed to anti-virus tools. During this time interval a new worm can infect many computers and create significant damage. We propose an innovative technique for detecting the presence of an unknown worm, not necessarily by recognizing specific instances of the worm, but rather based on the computer measurements. We designed an experiment to test the new technique employing several computer configurations and background applications activity. During the experiments 323 computer features were monitored. Four feature selection techniques were used to reduce the amount of features and four classification algorithms were applied on the resulting feature subsets. Our results indicate that using this approach resulted, in above 90% average accuracy, and for specific unknown worms accuracy reached above 99%, using just 20 features while maintaining a low level of false positive rate.