|You need to narrow down the model of the motherboard. There are multiple boards that may fit your incomplete description.|
That said, Vista SHOULD be able to supply a driver for that network chip.
Try downloading and running SIW to identify the motherboard model. Get SIW portable version (no installer) at the link below. After identifying the board go to the Gigabyte site and download to a different computer and transfer the driver/s to the computer in question via CDR/ flash drive, etc.
Gigabyte link below.
Upon further checking I don't think Gigabyte will have any Vista drivers listed.
When you view Device Manager are there any other devices not working properly?
I seem to recall that nvidia did not support Vista for nForce 3 chip sets.
That said, the network/LAN chip is probably a third party item. As such you should be able to get a driver from the manufacturer of the chip. SIW may yield the chip model.