Do the following steps in your target Windows Server 2008 .

To get the sidebar,

1. From a Vista SP1 x64, copy in the content of C:\Program Files\Windows Sidebar. And place the folder in the same location.
2. Run sidebar.exe /regserver
3. Register (regsvr32) the 2 sidebar dlls, sbdrop.dll and wlsrvc.dll
4. Clone the sidebar registry settings of the Vista box, HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Sidebar\Settings, to the target Windows Server 2008 registry.
5. Run C:\Program Files\Windows Sidebar\Sidebar.exe to bring up the sidebar and set the properties to start Siderbar when Windows starts.

Now to get the Aero,

1. In Server Manager, add Desktop Experience feature.
2. In Services applet, set the service, Themes, to autostart.
3. In Personalization, set Theme to Windows Vista and Windows Colors and Appearance to Aero.

Then spend the next hour to add/remove all the gadgets you like/dislike, which is the fun part. After all is done, here's what a Windows Server 2008 desktop may look like.

Source- Technet Blog

Thanks Ankur.

Gr8 tips & tweaks !