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Based on a set of Department of Defense specifications for trusted systems known as the Orange Book, the Common Criteria certification doesn't guarantee that an operating system or software application is bug-free, but that the development and support processes that created and maintain the product meet a certain level of standards.
Windows 2000 was certified at Evaluation Assurance Level (EAL) 4, which the Common Criteria Web site defines as "methodically designed, tested and reviewed." The certification is the highest level that can be attested to by a commercial laboratory. The top levels of certification--EAL 5, 6 and 7--must be attested to by government agencies.