I've just done some experiment, and it may be that this is related to the
use of Microsoft Expression Web. Anyone heard of a problem here?

(cross-post added to Vista Networking)

If you have already visited a site once with a particular instance of IE
(e.g. iexplore.exe task) it apparently avoids both a real DNS lookup
and a dnscache lookup.   You can prove this by doing a   /flushdns  during
an active session, do a new transaction on that session and then do
a  /displaydns.   Assuming only sites already accessed by that task
were reaccessed the  dnscache  may be empty.  To make sure
that your testing involves a real HTTP transaction (and just not rendering
from the TIF) you should use either a noncacheable page or a page
you have never visited before (or do a Ctrl-F5 Refresh but I'm not sure
if that maintains the task's DNS cache.)   Another complicating factor
would be expiry of the refresh time for the cache's entry  (ref.  KB263558)

Therefore a better test to be sure DNS was not involved would be first
to flush the dnscache and then open a new instance of the browser
e.g.   Run...  iexplore.exe  -nohome -extoff (and verify using Task Manager
that it is new) then try to use it to connect to your problem site.
(Note that you have to press the Stop button to get out of the  -nohome state.
BTW the  -extoff  switch may be unnecessary but it would test at the same
time the hypothesis that your symptom might be due to interference from
incompatible third-party programs.)   Then check with  /displaydns if there are
new entries in the dnscache.  Unfortunately, the results if there
aren't any entries could still be ambiguous and you would need to do
a full packet trace to actually prove whether IE was trying to get that far.
E.g. it isn't clear to me excactly when the dnscache gets updated.
If it is only updated after a successful connection request,
externally it could still look as if the  DNS  lookup was failing.

Starting a new task might change your symptom too.   ; )

That seems a trifle extreme.   ; )    So you have closed all instances of IE
and  tried using new tasks?  Is a proxy involved?  (Local or remote.
Also you might not be aware of filtering and relaying maintained by
certain AV products or unknown malware.)   In any case verify using
netstat  -ano  that all traces of old HTTP connections are gone.

If you want to try to diagnose this I would start with  FiddlerTool.
E.g. it will show you if a  GET  request is being issued and if there
has been a response from the host server.  If  Fiddler shows that
responses are pending it would be clearer evidence that IE is not
the cause of the blockage (regardless of other browsers being able
to function at the same time.)

I don't have your OS, so I don't know if any better user diagnostics exist yet
in it.   E.g.  the  Diagnose Connection Problems...  tool at least in XP sounds
like a good one but IMO it doesn't really make clear where a blockage is
occurring.   Cross-posting to Vista Networking for assistance.

HTH

Robert Aldwinckle
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