MS: How to Extend the Windows Server 2008 Evaluation Period

How to Extend the Windows Server 2008 Evaluation Period

When the initial 60-day evaluation period nears its end, you can run the Slmgr.vbs script to reset the evaluation period. To do this, follow these steps:
Click Start, and then click Command Prompt.
Type slmgr.vbs -dli, and then press ENTER to check the current status of your evaluation period.
To reset the evaluation period, type slmgr.vbs –rearm, and then press ENTER.
Restart the computer.

http://support.microsoft.com/kb/948472