Microsoft allow you to have an unlimited number of testing during the evaluation period of the Windows Server 2008. You can use that evaluation version for 10 days or 60 days without any activation. When the initial 10-day or 60-day evaluation period nears its end, you can RESET the licensing status of the Windows Server 2008 evaluation with below steps :
1. In windows server 2008, click Start, and then click Command Prompt.
2. Type slmgr.vbs -dli, and then press ENTER to check the current status of your evaluation period.
3. To reset the evaluation period, type slmgr.vbs –rearm, and then press ENTER.
4. The command is a call to the “slmgr.vbs” script with the -rearm parameter, after that’s finished you will be asked to restart the server.
Hope this helps you evaluate your copy of Windows Server 2008