Once you have the new Aero engine enabled on your build of Windows codename Longhorn, you can enable additional transition effects that are currently turned off in builds. Follow the steps below to add some additional desktop and explorer effects:
1. Open up regedit.

2. Navigate thru HKEY_Local_Machine, Software, Microsoft, Windows, Current Version, and Explorer.

3. Create a new DWORD and call it MILDesktop.

4. Set the value of MILDesktop to 1.

5. Create a new DWORD and call it MILExplorer.

6. Set the value of MILExplorer to 1.

7. Exit explorer and restart.