Unauthorized access by intruders to computer systems is a pervasive and seemingly worsening problem. This research explores the implementation of the Intrusion Sensing and Introspection System (ISIS). ISIS is an Intrusion Detection System (IDS) implemented in a hypervisor, which gives it the advantage of good visibility of events occurring in the operating system but also isolates it from the operating system so that if the operating system is compromised, the attacker cannot tamper with ISIS. ISIS uses this isolation to increase detection accuracy by watching for the symptoms of a successful attack rather than the attack itself. We introduce a symptom called a primary backdoor, which is the first interactive session that an intruder gains after a successful attack. In experiments with various exploits, as well as honeypot machines placed on the Internet, we were able to achieve detection of a variety of different attacks with very few false positives.