"You might not have permission to use this network resource"

Another one of these problems -- I've found a few of these threads, but my situation seems to be a little different.

Here's the deal. I have a small home wireless network with two Vista machines: a desktop (let's call it ZEN-DT) running Home Premium and a laptop (ZEN-LT) running Business. Both are running software firewalls and both's IPs are in the other's Trusted Zone.

For months I hadn't had any problems whatsoever accessing one from the other across the network. Then I began to have minor problems accessing the laptop from the desktop after I had to reinstall Vista on the desktop a little while ago. But now for some reason I can't access the laptop from the desktop AT ALL. (Though I can continue to access the desktop from the laptop.)

The desktop still seems to know about the laptop on the network. I go to Windows Explorer and I can see the laptop. But when I try to access it I get the "\\ZEN-LT is not accessible. You might not have permission..." etc. message.

The thing is, I can successfully ping ZEN-LT both by IP and by name. From the laptop, I've tried shutting off the firewall to the Trusted Zone entirely, but this didn't change anything.

I operate on both machines exclusively with an account with the same username and password. The Sharing and Discovery settings on both machines are all set for sharing (everything on but "Media Sharing"; I tried setting "Password protected sharing" both on and off and there was no difference) and the network is private.

Does anyone have any ideas what could be wrong? This is driving me nuts!