IE6 stopped connecting to websites at 3 a.m. 30 March. Help!

[Guest]

My copy of IE6 (6.0.2900.2180.xpsp2_qfe.061219-0311) was working fine last
Thursday night, until about the time Microsoft update runs on my system.
After that, it failed to connect to any website at all, always giving a "the
page cannot be displayed" error.
It definitely seems to be an IE problem; other internet services (email,
software updates etc) and Firefox (which luckily I had installed
just-in-case) are unaffected, and IE is fine with local content.
The behaviour is the same whether I connect to my local network by wifi or
ethernet. However, if I change my LAN settings from Automatic and specify the
network address of my router as proxy server, different sites give different
error messages, as follows:
"Transfer interrupted! Server Error: 503 Service Unavailable. SSI Error,
some SSI statements are not replaced!" or
"Hasbani Web Server Error Report. Server Error: 501 Not Implemented. No RPM
for this combination of URL and method."
I run Norton Internet Security, and it did a liveupdate at about 1.30a.m, so
that might be relevant, though my impression is that IE continued working
later than that.
I've tried clearing out all cookies, reverting to default internet options,
getting Windows File Protection to check and if necessary restore all
protected Windows files, and replaced the ie.inf file with one from the XPSP2
upgrade - all to no avail.
Can any wonderful person give me any idea what might be happening, and what
I should try next? (Please don't say IE7 or Firefox; I have specific reasons
to stick with IE6)
Many thanks

 

[PA Bear]

Does the behavior persist if you eliminate the router and connect directly
to the modem?

 

[Guest]

Thanks for the suggestion, but that's not an option - the ADSL modem and the
router are combined in the same box.