This is such a random problem.
Just setup a network for a friend of mine, he has two computers and a Virgin cable connection.
So I got an old Linksys wired router off eBay for £15, shoved that with their current modem (old Motorola Surfboard SB4200) with ethernet out into the modem ... One of the computers work fine, as well as my laptop, however one of the computers can connect to the network, access local shares, and ping the outside world (google etc.) however it just times out when it tries to access them in IE, Firefox, Safari. What is weirder is if I get the dodgy computer to create an ad-hoc WIFI connection - my laptop will connect to it and can browse the internet perfectly through this dodgy computer :S
The computer is a G5 iMAC, running 10.5.4 ... any ideas? I'm stumped apart from re-installing OSX