D-Link DIR-100 Internet OK, LAN not working

I bought this DIR-100 the other day to replace my old DI-704 (which was to slow). The internet connection works excellently from both our computers (Windows XP SP2, Windows Vista) but no connection between them (file sharing, printer sharing etc) - they don't see each other!

I seem to have tried everything (router settings, computer network settings, installing the LLTD Responder on the XP computer etc.etc.)

Will be extremely obliged to you if you can solve this one.

Greetings from Lund, Sweden,
Stig