Make sure you're using Desktop Manager 4.2 SP1. It's the only version that's truly Vista ready and compliant with systems patched for DST 2007.

Uninstall any older versions of Desktop Manager you have installed before installing this one.

The file you download is a self extracting EXE. If you can, download the file and extract it to a directory. I use C:\WINDOWS\Options\BBDesktop42SP1, and run BlackBerrySetup.exe to install. Make sure you right click on BlackBerrySetup.exe and select Run as Administrator when you run it.

I've had no problem setting up both an 8700 and an 8800 as a tethered modem using a USB cable under Vista. I have no experience using Blue Tooth as a data connection, but if you have it working with Desktop Manager, it should work as a modem.