3D desktop for Windows 2000/XP using the application Madotate. Hope that the below screen shot says everything.  To install it, just create a folder on your hard drive, unzip all files and run it. It also includes a plugin that adds a button on every window to make it 3D/2D.

But just want to tell you that this is not equal to minimizing the windows. This will not free memory. check this post to know about  Memory management for windows users.

You can get Madotate from here