Thanks for the replies.
The problem is now fixed but it was a pretty laborious process to diagnose. Luckily I had GadgetMaster on the end of phone, since my windows expertise pretty much stops with Windows 2000
For some unknown reason, the windows drivers went a bit awry. They'd lie to the device manager that all was well but in fact they'd literally shut down the NIC. And by shut down, I mean totally, to the extent that a reboot didn't reset them, even if you went into the BIOS and tried a LAN check, it couldn't detect a link. The link would simply stay down, even when the machine was powered off. Whatever Vista did to it was only fixed by rebooting into Linux or by powering down the system and disconnecting it from the mains supply. As soon as Vista started booting again, the link would go down again and wouldn't come up until you rebooted into linux once again.
We tried doing a system restore to the point prior to the last windows update, but alas that didn't fix the issue.
So, we tried rolling back the NIC drivers and as if by magic, the problem was solved, though quite bizzarely, the "older" drivers had a more recent publishing date.
It gets a bit stranger here since I decided to "undo" the earlier system restore, with the intent of getting it back up to date in every other respect, expecting the LAN connection to fail again and fix it by rolling back just the NIC driver. However, that last bit hasn't seemed necessary this time.
I can only assume it wasn't the driver per se that was at fault but some how an associated configuration file got damaged.
Windows, eh?
