Internet Strom Center shows a rather interesting fact: an PC with a new OS installed with out any patch can not survive for 5 minute averagely.

“The survival time is calculated as the average time between reports for an average target IP address. If you are assuming that most of these reports are generated by worms that attempt to propagate, an unpatched system would be infected by such a probe.

The average time between probes will vary widely from network to network. Some of our submitters subscribe to ISPs which block ports commonly used by worms. As a result, these submitters report a much longer ‘survival time’. On the other hand, University Networks and users of high speed internet services are frequently targeted with additional scans from malware like bots. If you are connected to such a network, your ‘survival time’ will be much smaller. “

For more, click: SANS