I have 2 Windows XP computers connected on a wireless network via a router. Both machines can see each other's shared folders and on machine 1 I can connect to the Internet without a problem. The connection on the other machine OTOH is so sssllooowww that Firefox (or IE8 for that matter) never manages to load a page ("The connection was reset" after 15 minutes or so).
PC 2 can ping the router and the other machine successfully, and also load the router's setup page.
The software (OS + apps) was reinstalled today, and Firefox did work properly after installation.
Fixed! Installed an audio(!) driver and everything is peachy again (for the time being). Question can be closed AFAIC.