how to use windows aero in server 2008?

It will depend on whether the graphics card you have on your machine has drivers for Windows Server 2008, and if it supports Aero. But you can get some of the visual effects and programs as you would get on Windows Vista by installing the Desktop Experience feature using the Add Features wizard in Server Manager.

You also need to enable the Themes service to start automatically, change the Theme under control panel->personalization to "Windows Vista" and finally change window colors and appearance to Aero. Moreover, the drivers you'll need are likely at this point to be Vista drivers because most cards that support Aero aren't going to have Server drivers and those that will probably aren't available yet.

I have the same problem: there is no Vista theme availible. How to enable the themes service?Thanks.

Just like any other Service. Go to the Services "Administrative Tool"/MMC Snap-in and change the Themes service to automatic startup. And start it as well. -Rob