Are you running this on a user account, non-admin, there must be something missing on Vista Business that may be causing the problem, as i am not sure of this do you have WMP installed (should not affect but you never know).
Vista Ultimate has no problem running MP here.
From the start menu can you select MPTestTool and reproduce the error, upload produced zip file using the Forum go advanced + feature.
This will hopefully point to where any system error may be.
Vista Business isn't the problem either. I tried to install mediaportal on my other computer and it works fine. I can also cross out Netframework 3.5 as the problem occurs with Framwork 3.0 and/or 3.5 installed. I even installed DirectX 9c alongside to DirectX10 to no avail.
But there it a glimpse of hope: After installing directx 9c i started dxdiag and it froze while checking for directinput. As soon as I get home I will try to fix that problem and update you on the forth come.
I finally fixed the problem: The cause is the driver for the "Media Centre Controlboard" (SL-6495-SSV) from Speedlink. There are two versions of the driver, one that came supplied with the Controlboard and one "Vista Final" Driver from the Speedlink Website. Both actually wreak havoc. Dxdiag won't show any Info on the DirectInput Tab and Mediaportal won't start up. There are probably some more problems caused but at least I can confirm those.
Once the driver is installed, Vista will show a "MediaCenterDriver" under “Installed software”. Deinstall it and Mediaplayer will start normally. Btw: The SL-6495-SSV will work well without that driver installed.