Thanks for the links.

When I try ipconfig /flushdns it says (how surprising):
Could not flush the DNS Resolver Cache.

Next I tried ipconfig /registerdns. It says that the action has been initiated, and that any errors will be reported in the event log. I hadn't thought yet of looking there, but it contains some interesting errors (not related to the registerdns), which occurred when I started the PC (I have an ADSL connection to the Internet, and it seems to function without problems).

The first one: The Point to Point Protocol module C:\WINNT\System32\rastls.dll returned an error while initializing. The network request is not supported.
The second (same time): The Control Protocol EAP in the Point to Point Protocol module C:\WINNT\System32\rasppp.dll returned an error while initializing. The network request is not supported.
The third: Remote Access Connection Manager failed to start because the Point to Point Protocol failed to initialize. The network request is not supported.
The fourth: The Remote Access Connection Manager service terminated with the following error:
The network request is not supported.

I haven't changed any settings recently, but about one week ago I had severe problems because of a 0x1E stop. At some point a lot of Windows files were repaired via the recovery console. Perhaps there are still a number of Windows files in a not so good condition?!