A hard disk in my wife's laptop was damaged. Windows 7 did not boot. After confirming the damage from Linux (openSUSE 12.1 install CD and AVG Rescue Disc) we called Dell's Support and got a new disk. There was no system on it and I found our "rescue" DVD did not work, so I tried to install Linux on the empty disk.
I already had openSUSE 12.1 NET installation CD, so I tried it. The problem was I was not able to connect to the internet: I used a RJ 45 ethernet cable to our D-Link router DIR-652. I was able to ping the router, I was able to browse its web interface, I was able to ping other computers in LAN, but when trying to connect to the WAN, there was no success. When pinging for a long time, one packet got through occassionaly, ping reporting about 97% packet loss. I gave up.
The next day a guy from Dell brought us Windows 7 installation disk. To my surprise, after installing the system, I was not able to connect to the internet either - and the symptoms were exactly the same as before in Linux. I was not able to use wireless connection, because the driver was not present on the installation disk (nor on the Dell drivers disk, either). I tried various things: switching the cables, changing the setup of the router, but to no avail.
Then, at last, I tried to bypass the router and connected the laptop directly to the cable coming from the wall. And lo and behold! I was connected. I ran the updates, got the driver for wifi and prepared the computer for normal work.
But I still wonder why it did not connect to the internet through the router by the ethernet cable.