Thanks for the reply UK Bob!

1. Sorry, I forgot to mention that of course I tried to switch off the XP firewall, but I still cannot connect to LW
2. LW was working fine on my (old) system a few months ago, I had the same ISP.....Of course, it is possible that my ISP has changed something over the last couple of months (I will check Dutch fora for this).

Could my problem be related to Java? A couple of days ago I installed an older version of LW (I think it was 4.08 or something..). I remember during the installation there was something with Java, but I don't clearly remember what it was. Meanwhile I have installed a newer version of LW (4.12.11) (because I hoped that this would solve the problem), the installation went smooth now, but LW is still not connecting.
Is there a way to test if my problem is related to Java?

I've read something on this forum that was related to the online game Counter Strike: Source. I have this game on my computer, but I don't think this could be related to the problem now, could it...?