Microsoft Certifications do not expire. They simple lose value with time. For example, an MCSE on Windows NT 4 was all the rage in 1998. In 2009, that certification will not go far in a job interview. Microsoft publishes "upgrade" exams to help keep you up to date in your certification. The upgrade exams test you thoroughly on the new features of the OS. If you were starting for scratch or with a very outdated MCSE, you will have to take all the exams. This will not only test your knowledge of the new features, but also the basics that the certification requires.
Here is a little Q and A I pulled from a Microsoft site:
Q. How long will the certification be valid?
For our newest credentials, such as Microsoft Certified Technical Specialist (MCTS), Microsoft Certified IT Professional (MCITP), and Microsoft Certified Professional Developer (MCPD), the credential retires along with the product support for the technology being tested. The credential will still appear on your transcript but will be listed as retired. In most cases, an upgrade path (usually one exam) will be available for individuals who have that credential so that they can demonstrate their skills on the newest version of the technology without completing all exams associated with the new credential.
The legacy Microsoft credentials, such as Microsoft Certified Systems Engineer (MCSE) and Microsoft Certified Systems Administrator (MCSA) do not expire, but as Microsoft releases new versions of the associated technology, these credentials are likely to be valued less by the industry.
During a recent poll of IT hiring managers, 55 percent said that they consider employee certification as a criterion for hiring, and 63 percent of hiring managers said that they believe certified employees are more productive than noncertified employees.