this worked for me with WinXP: (for Win7 I had to use Toshiba Bluetooth Stack)
take a look at START->Settings->ControlPanel->System->Hardware->DeviceManager
a device with a question mark will be there for your bluetooth, click onto it and open the tab details, there you will find the VID and PID of the stick. Remember them!
open the file C:\WINDOWS\ServicePackFiles\i386\bth.inf (or make a search for the file: bth.inf)
in the device section are the supported Bluetooth devices with their VID & PID listed
just pick an existing manufactor (Cambridge) and enter below:
CSR Bluetooth Generic= BthUsb, USB\Vid_044E&Pid_3001
CSR Bluetooth Generic= BthUsb, USB\Vid_044E&Pid_3001&MI_00
The VID and PID (and sometimes the Revision too) must be changed to the ones you just wanted to remember.
save the file and close it
now make a reboot
if the driver installation asks about missing files then point it to your bth.inf
after the driver installation everything should work! now you are using the Microsoft Bluetooth Stack.