Okay,
Did some more digging here tou do have a homepage and searchpage hijacker there (O8 - Extra context menu item: &Search -
http://bar.mywebsearch.com/menusearch.html?p=ZRxdm075_ and O2 - BHO: (no name) - {00A6FAF1-072E-44cf-8957-5838F569A31D} - C:\PROGRAM FILES\MYWEBSEARCH\SRCHASTT\1.BIN\MWSSRCAS.DLL)
It can be uninstalled via Control Panel > Add/Remove Programs. It will show up as "My Search Bar" or "My Web Search".
Please also run the following program from here...
http://www.webroot.com/services/spyaudit_03.htm
The thing about this as you already know - is that this is a page which NTL decides to show when it detects something wrong. It could be that you have a faulty modem, it could be that your AVG isnt picking up a virus too, the trouble is - that it could also be there is nothing wrong and you get this for no reason.
I would still suggest that you try a different virus checker - also in your firewall, look at the logs and see if you can see anything strange that might be generating traffic either constantly or in bursts.
Sorry i cant really help more than this though.