just to confirm...when you've swapped from usb to ethernet, that you have shutdown the pc, removed all of the leads to the cable modem, including the catv, leave em out for 10 or so seconds, reconnect the catv, ethernet cable, then power. wait for the sync and rdy to lock on... then restart the pc. should be job done!!! there's nothing on the Broadband installer cd that you need to run the connection via ethernet, the modem drivers on the cd are for usb connections only.
if your gonna go down the removal of NIS route, and have any problems removing it.. there's a removal tool on the symantec website. there's a list of tools available for various symantec products, listed here:
http://service1.symantec.com/SUPPORT...01092114452606
I'd agree with mick on the firewall and AV recommendations... there's also Avast antivirus and Zonealarm firewall to give you a few options. might also be worth running a few antispyware programs like the ewido security suite (free for 14 days), adaware, spybot, ms antispyware and praps CCleaner. once you've downloaded, installed and updated em.. run them one at a time in safe mode.
and if your gonna try a speed test, it might be worth running one in normal mode first... then again in safe mode with networking... and compare the results.
obviously still could be summit wrong with the cable modem itself... which would need to be sorted out by NTL.