It is worth checking whether or not your firewall is turned on on the Vista machine - it's a long shot but worth a look as I've seen curious network problems solved by simply turning the Windows firewall off. In your case this is perfectly fine as it sounds like you have a firewall built in to the router.

If the retailer has tested the card and confirmed it works, is receiving an IP address and so forth, then there isn't much else it can conceivably be short of an intermittent cable or card fault which is again highly unlikely.

Let me know how you get on and I'll do some digging - I must admit I'm running low on ideas given the symptoms.

John