Edit: Hah! As it turns out, Vista ended up finding an update to the driver through Windows Update. It was considered an optional update, but installed the 3Com Gigabit LOM (3C940) which has been working fine as well. So if you want the 3Com driver, just install the Marvel one temporarily (so you can get on the net), and run Windows Update to get the 3Com driver.
My ASUS P4P800 happened to come with an onboard LAN which Vista doesn’t supply drivers for. Not only that, but the XP driver listed by ASUS doesn’t show as being Vista Compatible (although I didn’t try to force it). So what to use? Well, quite some time ago when I was playing World of Warcraft, their built-in updater/patcher used to whine about the network card driver not being up-to-date enough. A Blizzard employee did some looking around for another individual, and found
that a driver that would work (and keep the blizz downloader happy) was a generic Marvel driver. As a side note, if anyone wants that XP driver (for WoW or another game), leave a comment or send me an email and I’ll be happy to send it to you. (Edit: I’ve put them up on the site. Download for Windows XP/2003 or Windows XP 64-bit.)
Anyway, getting back on topic (Vista), what worked for me was the built-in Vista network driver called “Generic Marvel Yukon Chipset based Ethernet Controller”. For those who need some instructions to install it, Simply go to the Device Manager (found in Control Panel), right-click on the Network Controller, and choose “Update Driver Software”. Then choose “Browse my computer for driver software”, followed by “Let me pick from a list of drivers on my computer”. Under the Manufacturer section, choose “Marvel”, and under the Network Adapter section, pick “Generic Marvel Yukon Chipset based Ethernet Controller”. It may give you a warning about the driver, but I simply ignored it, let it install, and the network came up and was good to go.
This was using Vista RC1 Build 5728. It would have of course been nice if ASUS got a hold of some new drivers and put them on their site, but failing that, this is probably the next best thing, and is at least a Vista-certified driver (although not a driver certified specifically for the 3C940).