- Deactivate Bluetooth in the Asus-Tray-Utility
- Open C:\WINDOWS\INF\BTH.INF with the Editor.
- Look for:
[Manufacturer]
%Microsoft%=Microsoft
ALPS=ALPS, NT.5.1
Belkin=Belkin, NT.5.1
...

- Now add "Asus=ASUS, NT.5.1". It should look like this:
[Manufacturer]
%Microsoft%=Microsoft
ALPS=ALPS, NT.5.1
Asus=ASUS, NT.5.1
Belkin=Belkin, NT.5.1
...

- Now look for "[Zeevo.NT.5.1]". It should look like this:
[Zeevo.NT.5.1]
Zeevo Bluetooth Solution=                        BthUsb, USB\Vid_0b7a&Pid_07d0&Rev_0126
Zeevo Bluetooth Solution=                        BthUsb, USB\Vid_0b7a&Pid_07d0&Rev_0133

- Beneath it, add following:
[Asus.NT.5.1]
Asus BT-253=                        BthUsb, USB\VID_0B05&PID_B700

- Result:
[Zeevo.NT.5.1]
Zeevo Bluetooth Solution=                        BthUsb, USB\Vid_0b7a&Pid_07d0&Rev_0126
Zeevo Bluetooth Solution=                        BthUsb, USB\Vid_0b7a&Pid_07d0&Rev_0133
[Asus.NT.5.1]
Asus BT-253=                        BthUsb, USB\VID_0B05&PID_B700

- Save and leave the editor
- Now it depends wheter you did install the Widcomm Software or wheter you start with an fresh Win XP Installation

With Widcomm-Software:
- Activate Bluetooth in the Asus-Tray-Utility, Bluetooth Symbol should be white
- Go to System, Software and remove Widcomm-Bluetooth-Software
- At the end, system want that you deactivate Bluetooth - do that via the Tray Utility
- Than, it wants to reboot - DON'T do that.
- Activate Bluetooth again in the Asus-Tray-Utility
- Windows should start installing its own Bluetooth Stack
- Reboot after that, you're done

Without Widcomm-Software:
- Just activate Activate Bluetooth in the Asus-Tray-Utility and it should start installing
- If not, go to the Hardware Manager and "search for changed Hardware"

Internal Notes:
C:\WINDOWS\INF\BTH.INF
rundll32.exe bthprops.cpl,,BluetoothAuthenticationAgent
bthprops.cpl

I found it btw in an german forum and did roughly translate it, because I found it very useful as I only use Bluetooth for s25@once with my Handy.