I just noticed your last sentence about browsing when it seems to happen. What browser are you using? If other than IE, it could be something odd going on with HT and some unusual browser type (Opera, Firebird, etc). Or, could be browser support software doing it like any software firewall, or other apps in your system tray. Maybe even some usual AV software. Disable ALL of these if any and see what happens. Also, if you're on a dialup, or are using software with your broadband, that may possibly be doing it. You don't need ISP software with broadband, and even on a dialup 99% of the time you don't need it. Also don't rule out any drivers for any broadband modem (if internal or USB since ethernet doesn't need drivers).
FWIW, I use IE, XP Pro, Sygate, PCcillin, SpyBot's new v1.3 w/"TeaTimer" active, SpywareBlaster*, Logitech Mouseware, PopUp Stopper Pro, IE AutoSizer, IE Privacy Keeper, all running in the background and they don't cause these HT problems you mention on my PC. (*SpywareBlaster isn't running in MSCONFIG or cont-alt-del, but it has active protection due to registry blocking tags). This is on aDSL and an external ethernet modem w/router.