Hi gazza,
It's unlikely ntl support will help, as it sounds like this is more of a PC issue than an issue with the broadband connection (though I can't rule it out - seems strange that it started after the box swap). Can I ask what browser you're using (Internet Explorer, Firefox, etc.)?
If you're using IE, this page from the MS support site might help - although it refers to IE5, it may apply to IE6 as well:
http://support.microsoft.com/kb/232484/EN-US/