My internet signal comes via a DSL modem/router and a switch which feeds an old Windows XP and a newish Windows 7 computer.
On the Windows XP computer I have no issues.

On the Windows 7 computer, some domains work without problem as well, for example http://www.google.com/reader/view/ or heise.de.

Others, like google.de do not work at all. It tries to connect for some time and then says "Server nicht gefunden" (Server not found).

At a third kind reproducably only a certain part of the information arrives.
An example is gizmodo.com. I guess some part of the info comes from one domain and another part from another one.

The issue is exactly the same on FireFox, IE and Chrome. I temporarily stopped AVG and Windows firewall and it did not help.

So I think that Windows itself is keeping some blacklist. Any ideas?

Try cleaning your DNS cache -

Open a command window as administrator (type cmd in the search bar, right click cmd.exe, select run as administrator).

Then type "ipconfig /release"

then "ipconfig /flushdns"

then "ipconfig /renew"

close command window.

Try browsing.

Excellent, that fixed it, thank you!

Out of curiosity, how does the DNS cache get corrupted?

A lot of ways -- glitches, malware, etc. It may also have been working fine; if the site was actually unreachable or if your DNS server was having a fit at the time the domain name query was cached, you'll get errors until the entry expires (or is flushed).

Ok, I see, thanks