Netgear Gigabit NIC won't work right

I'm running a mostly-XP LAN, with a number of machines connected together
through a gigabit switch. Everything runs fine, including my new Sony Vaio
SZ440. However, the Vaio only works at 100 mbps and if I use the built-in
NIC (and I have the "slow copy across network" problem, though it sees all
my machines and can access their file systems).

I have a couple of Netgear GA511 gigabit NIC cards. One is running in a
Toshiba Satellite (an XP machine) and it works fine. I tried the other in
an older Vaio (a V505ax), also an XP machine, and it works fine as well.

However, neither card works correctly with the new machine under Vista.
Vista completes the install, device manager shows that it's working
correctly, and I can ping everything on the LAN, including my ISP's DNS
server. However, neither Explorer, FireFox or any other software can access
the internet, and status reports that the NIC is running at only 10 mbps
(that's not a typo). I've tried uninstalling and re-installing the NIC,
even re-installed Vista from scratch, but that didn't help. I've adjusted
the advanced settings for the card, forcing both link and NIC speed to 1
gigabit (and in various combinations), but that doesn't work either. I've
tried disabling IPv6, but that makes no difference, either.

I've run out of ideas, and would welcome any suggestions.