Windows Server 2008 gives you 30 days to activate the OS. If you change hardware on the physical server, Windows may require re-activation. You will get 3 days for this activation grace period. For those with evaluation copies, your grace period is 60 days. You can re-arm the grace period by following the procedure below.
1. 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. Restart the computer.