Yeah, I've seen it - posted about it a while ago. It's Norton, specifically the SYMPROXYSVC.EXE process. Never seen any other site trigger this, but Symantec's knowledge base claims it is a problem that was fixed some time ago. Symantec's Desktop Firewall doesn't do it.
FWIW, the system is XP/Pro with NIS2002, both bang up to date. IE6, but seen it with IE5, 5.5 and Netscape (forget which version).
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