Proxy authentication required problem

Hi

My ms access app is posting to a web site using ms internet transfer
control. Client pcs where app is running are connected to a small business
server 20003/ISA Server 2000 server. The app works fine on all pcs which
have win xp sp1 or earlier. However it does not work on a new pc which came
with sp2 pre-installed. I have disabled the sp2 firewall but the app still
does not work. The error I get is 'HTTP Error 407 - Proxy authentication
required', ISA Server can not retrieve the web page etc.

What can I do to make the app work as it is working on win xp sp1 pcs?

Thanks

Regards