Windows Server 2008 doesn’t have the ‘Windows Experience Index’, but your system is checked though. If you system is fast enough and you have the Desktop Experience feature installed, it will automatically enable Aero.
I think the Experience Index doesn’t have anything to do with the Slideshow function. Just checked the Slideshow function in a Vista x64 system in Hyper-V and it seems that the Slideshow feature is integrated in explorer.exe. I think it will just be a matter of modifying/adding the right registry keys to get it working…
If you really want to see your experience index, maybe it is possible to copy the tool winsat.exe from a Vista x86/x64 machine to check your systems score.
