Web page through VPN awfully slow with only Windows XP Professiona

Hello,

I am really having a nightmare with Windows and IIS 6.0:

* I have an intranet solution developed in ASP.NET using VB.NET 2005.
* This intranet asks for Windows Authentication using simple text
authentication.
* I have a beautiful 3Com Router / VPN Server that allows me to log on into
the LAN using VPN.
* The idea is that users on the "go" can log on into the VPN connection and
access my intranet solution.

But,

* If the user uses a laptop with Windows XP Home Edition, the comunication
is lovely fast.
* However, if the user uses a laptop with Windows XP Professional that was
once part of the company's domain, the web page takes FOREVER to load, on
every d##n click, and the odds of loading a page is about 70 %.

I already:

* Tested different computers and the result is the same: The ones with
Windows XP Professional that were once part of the company's domain take
FOREVER to load.
* The Internet Service Provided was the same in all my tests: I even used
the same network cable to test and the same Jack on the same remote network.
* I disabled all antivirus, all Internet Explorer add-ons, all odd apps...
even one computer was almost new, and the result was the same.
* And of course, the internet bandwith was lovely big: 1 MBps (long live
Bell Canada).

The only thing that I haven't tried is:

* To Format all of them and downgrade from Windows Professional to Windows
XP - that's going to hurt.

Any help is greatelly appreciated!!!!

Best Regards,

Tarh Ik