I am sorry. I've made the common mistake of not checking everything when I first replied. The Xerox Phaser 8400 is not compatible with Vista (natively) so you need to make certain things by hand.
First, download the correct drivers
which can be found here
. Get the ones almost at the end of the page called "Windows PostScript PPD Driver Files - 32/64-bit
". Now, you have to erase whatever is left from your prevous installation attempts.
Go to Printers, richt click on any printer and go to Ports. Look for the "Sandard TCP/IP port" that's pionting to your printer's IP port. If it's there, select it and eliminate it. If it's not, better
Now install it by using your new drivers and remember the path for the installation so you can re-try it by hand using the method I posted above. As usuall in Vista, run the installation as an Administrator (right click -> Run as Admin.)
Hope it helps,