Help, I am confused about drivers for the A7N8X USB Drivers.

Let me explain:

I am one of the unfortunates who still doesn't have broadband in the
area, so I'm still struggling along with dial-up. I bought a BT
On-Air 1800 USB DECT modem (USB powered) which was working fine until
I changed my mainboard for the above.

Now I will get a problem pretty much every time I turn the machine on.
When I dial up to the Internet sometimes the port to the modem will
not open and the connection fails, this means that the dialup process
will hang until I unplug the modem and reconnect it. If I then retry
the connection it will usually work.

I have tried removing and reinstalling the modem drivers, I have tried
changing the mainboard USB drivers from the default XP SP1 to the
nVidia ones, but I still see the problem.

My setup:

ASUS A7N8X Deluxe
AMD XP 2000+ (Palomino core I think)
ASUS 8200 Pure VGA (nVidia GeForce 3 ti 200)
2 x 256MB Geil PC-3200
2 x IBM 7200 40 Gb
Pioneer DVD-ROM
Pioneer DVD-R

Everything is stock, no overclocking, although the memory is running
at 200MHz with 6-3-3 2.5T as specified.

I have tried the performance setting at Optimal and Aggressive.

I have also tried making the USB ports 1.1 only

All my other USB devices work as expected.

I have checked for updated drivers for the modem - nothing.
I have downloaded and installed the USB 2.0 drivers from the ASUS
site.
I am running the ASUS 1.16 main board drivers

Previously I was running a Shuttle Ak35GT/R board that only supported
USB 1.1, but I never had a problem with the modem.

If anyone can suggest a solution to this little problem I would be
grateful, but the thing I'm after with regards to the USB drivers.

It mentions in the USB driver install that XP SP1 supports USB 2.0 out
of the box.

I checked the Device Manager list and it lists the USB devices like:

Universal Serial Bus Controllers
- Standard Enhanced PCI to USB Host Controller
- Standard OpenHCD USB Host Controller
- Standard OpenHCD USB Host Controller
- USB Root Hub
- USB Root Hub
- USB Root Hub

I have replaced the Standard Enhanced PCI to USB Host Controller
driver with the nVidia one, but I can't change any of the others
(basically they report being unable to start if I update those
drivers, so they are back to the standard issue ones)

Should I be replacing the MS drivers with the nVidia ones or not?

I know this is a long one, but I am really struggling with the
solution.

Please help.

Mark