tracert example.com OK, loading example.com intermittently NOT, even w/o firewall

I'm having a strange, intermittent network problem on a Dell XPS running updated Vista.

It'll sometimes work for hours, then suddenly not at all. When it stops working, I can still do

Code:

ping example.com

and

Code:

tracert example.com

; however, connecting to MSN or to websites using FF or IE does not work, I just get "Waiting for example.com ..." forever (no error messages though).

Vista does not find any problems with the connection, and on Repairing it arrogantly tells me that the connection is now working, which is wrong.

The problem occurs both on local networks and with wireless networks (excellent signal strength, other computers worked fine on all tested networks).

Disabling the firewall (ESET) has no effect.

Disabling TCP auto-tuning has no effect either.

There are no proxy settings on this or any other computers on the networks.

Does anyone have any clue what to do? All the troubleshooting guides I find tell me it should be working since tracert is working and I don't have a proxy or firewall set up...