Establishing a network with Vista

I recently got a Dell Dimension E520 with Vista Home Premium pre-installed, I’m having massive problems creating a network (which I know, isn’t new) and I can’t figure out what the is wrong. So far it’s stumped two call-in centers, both of which I spent hours talking to. I’ve contacted so many people about this and I can’t find any help. I can connect to the network created absolutely fine and sites like Google and Yahoo will load fine. However anything else and it either takes five minutes or it never stops loading, this happens with both Firefox and Internet Explorer. I also can’t file share, or see anything on my downstairs computer at all (running XP, if that matters). Vista doesn’t seem to detect it all. Basically I’ve established the following; My Linksys WMP54G v4.1 PCI Wireless Card is fine. (new drivers installed, Vista compatible) My BT Voyager 2091 Wireless ADSL Router is fine. They are well within range of each other (signal excellent). There is no interference. Trying to establish a wired connection gives the same results. When I phoned BT they asked me to bring my new computer downstairs and plug it where my other PC is, with the BT Router plugged in too. The guy then setup a screen share and he tried to establish a connection on my PC himself, tweaking with everything possible. Didn’t work, and he didn’t know why. He said it was probably Vista because everything else was working fine. I saw somewhere in my research something that I can no longer find (it could be unrelated anyway) that basically gave instructions on how to alter something on Vista to possibly fix it, but it mentioned it’s very risky and could mean re-installing. Has anyone done this and found it to work? I’m sure the instructions were actually on an official Microsoft page actually. I think this problem is quite widespread, but despite that I can’t find a fix. ANY help would be much appreciated as I’m now well and truly baffled. Thanks,