NetBIOS is clearly the problem here.

I got a call today from a user saying that none of his mapped drives were showing up. After attempting to remote the PC and throwing a ping at it, it wasn't responding. These computers are notorious for never getting restarted (250+ day uptime), so the NIC probably took a nap. Restarting fixes 99.99% of problems.

Me: Doesn't look like it's connected to the network right now. Let's restart it just in case.

User: Well, I DO see that I have a TCP/IP address connectivity, but maybe it has something to do with the NetBIOS. I think it has to do with the Windows 7 conversion.

Ah yes. That damned NetBIOS.