Really slow Xbox 360 as Windows Media Extender

"By any chance, is your PC on a wireless connection? I have that setup and Media Center gives me exactly the same problem. When I actually get an audio or video file running it streams fine, but choosing menu items is agonizingly slow--it can take 30 seconds or more just to move from one menu item to another. It's a commonly reported issue.

Microsoft recommends connecting the PC directly to the router, but they say that the performance issues occur if both devices are wireless, and my 360 is wired directly to the router."

You could try Tversity to manage your media files for you. You install it on your Vista machine and it creates a Streaming Server that streams to your Xbox. I have had no issues at all with it and my media library is far bigger than 5GB.