KB article 948472 describes the process for Windows Server 2008 and includes instructions on installing an evaluation version of Windows Server 2008 from the standard media as well has how to set up a recurring task in Windows Task Scheduler to extend the evaluation period to 240 days.
This article describes how to extend, or re-arm, the Windows Server 2008 evaluation period. The evaluation period is also known as the "activation grace" period. These instructions apply to any edition of Windows Server 2008. This includes evaluation copies.
Evaluating Windows Server 2008 software does not require product activation. Any edition of Windows Server 2008 may be installed without activation, and it may be evaluated for 60 days. Additionally, the 60-day evaluation period may be reset (re-armed) three times. This action extends the original 60-day evaluation period by up to 180 days for a total possible evaluation time of 240 days.