I have a computer running Windows 2008 Server Enterprise (SP2). I have installed the Desktop Experience feature and activated the Themes service. From the Nvidia website I've downloaded the latest drivers for Windows Vista x64 and the they've installed without any issues (I assume they are the best for my OS, since Nvidia does not provide support for Windows 2008). When I activate Aero, the performance is really bad, I have issues even with the scroll in Firefox. If I run dxdiag everything seems to be fine, it says that hardware acceleration is on. Do you have any ideas on what is causing the performance issue and how to fix it?
One possibility: You have Hyper-V installed by any chance? If so you may want to check this TechNet discussion. Not good news as there isn't a full fix yet, although things have been improving since last year. It's an option between either removing Hyper-V (I know, usually isn't a good option), disabling Aero (not so bad an option), or getting another GPU (there are some reports of ATI cards being afflicted too). Intel, Microsoft, Nvidia and AMD having been sitting over this for well over an year. It will be fixed eventually.
Another: Is this a laptop by any chance? If so you may wish to turn off Nvidia's PowerMizer. Similar problems happen with users even on Linux with compiz.
Might it be because your server configuration is set up to give cpu priority to background tasks, not foreground tasks?
Just a word of warning for a server: aero can eat up quite an appreciable chunk of your cpu.
Peeter Van Ooijen has a nice blog post about getting Server 2008 up and running as a workstation. In the body of the text he speaks about getting his nVida video card running by using an inf file hack.
Good luck, hope this helps.