I wonder if there is any mileage in looking at this as a network problem? I'm thinking particularly of possible clashes to do with the Vodafone card.
One possibility that occurs to me is that Win98 has somehow got confused with its connection configurations. Are you able to login to the DG834G so as to access its configuration? If so, I'll be asking you to post some details of this, and of the Windows network configuration for comparison.
PS I'm sure it wouldn't have to be porn sites for malicious software to be involved, it turns up in lots of places...