Explorer not opening http sites while https sites work

Hi!

I am experiencing very weird behavior of my Windows 2003 server. When I try
to access a normal website, such as http://www.microsoft.com, the Explorer
simply sits there, the flag (top right corner) waves, but nothing happens.
Also, the normal message (bottom left in status bar) about "Opening ..."
does not appear. This is both with intranet and internet sites.

Now, if I access a secure site (such as https://localhost/exchange), it
works fine (both intranet and internet).
If I access the internet from e.g. Firefox, I can browse fine (except
Microsoft sites, such as windowsupdate, which check for Explorer). I do not
see any other network problems.

I have also reinstalled ("repair" option) Windows Server 2003, installed the
SP1, have all current patches (automatic updates running and working).
Explorer is showing Version 6.0.3790.1830.
It showed up last weekend and I have no clue what caused it (no wild
software installed or such a thing)

Does anyone have a clue what could cause this? I'm running out of ideas and
cannot find any idea that works in newsgroups or internet (except for some
threads ending in no responses). Any help greatly appreciated!

Cheers,
Hendrik

--
Hendrik G. Seliger
NL - Valkenswaard