MsMittens: I do have some antivirus tool on the windows machines and they get updated every week but there are always new worms before the updates and someway the gateway crashes even if the win boxes didn't get infected.

dcongram: The machine is not equiped with enough RAM to run NT4 sp6a proper. I have the antivirus tool from Ositis installed (running and updated) and I think there is some problem with this tool: it tries to remove the virus or worm and then I get problems.