If you need to install Windows Server and don't have a valid license, you can still work for 180 days. It's possible, because Microsoft offers a trial of Windows Server 2008.
After a fresh install you will be prompted to activate Windows (with a 30 days grace period). Microsoft will not send you a trial key, however. To extend the 30 days into the full 180 days you can "rearm" the grace period timer three times, each time getting another 60 days of free and legal usage.
To do that, run the following command:
after this, you need to reboot your server.
To check how much time is left on your current grace period, you can use: