Help Corrupted downloads on internet connection sharing

Hi All :-)
I have the following problem that I can't get rid of. It is making me
crazy, and if I can't find a hotfix I should abandon vista :-(
Every file downloaded through internet Connection Sharing from
computers on my LAN-WAN are currupted, whereas if I download them from
my PC (running vista ultimate x32) ,that actually shares internet
connection with a USB 56K modem, they are totally ok.
I can navigate internet fine from them with every browser, messengers,
and share files and printers fine. Only downloads from ICS are
corrupted with every browser and download manager!
If I try to share with WindowsXP (I have more operative systems
installed on my machine) all works fine.
I tried by uninstalling all firewalls, antivirus both on my machine
and the connected ones..with no results. I tried by disabling IP6,
enabling a lot of services related to network stuff that were
disabled.. nothing :-( I really hope an hotfix exists!
Thank U very much in advance,
Stefano B.