Windows 7 cannot map drive to Windows XP share

When I open the Network folder inside Windows Explorer on my Windows 7 computer I see a list of computer names (most of them are Windows XP machines). If I double-click a Windows 7 machine it opens that machine and shows all of its shares. However, if I double-click on a Windows XP machine it hangs for about twenty seconds then returns the following error message:

Network Error

Windows cannot access \\Windows XPcomputername

Check the spelling of the name. Otherwise, there might be a problem with your network. To try to identify and resolve network problems, click Diagnose.

Error code: 0x80070035 The network path was not found.

I can ping the Windows XP computer by name, I can connect to it from another Windows XP computer on the domain, I can double-click on the server machine and it will open correctly (running Windows Server 2003 for Small Business Server). I have been troubleshooting this for about 8 hours already (disabled my firewall, no help; disabled IPv6, no help; attempted to connect to IP address of Windows XP computer, no help; verified NetBIOS is enabled).

The thing I can't understand is that I was able to do this without any problem up until a few months ago. Something changed, but I don't know what. I have found several forums where Windows 7 users have reported this problem. Anybody have any new ideas?

UPDATE: I am able to connect to Windows XP computers on a different domain. (I do consulting work and I am a domain admin for several of my customers.) This makes me think that there may be some difference in network policy settings that is causing problems in Windows 7 <--> Windows XP communication/authentication.

UPDATE: The domain where I can connect to Windows XP computers uses DHCP. The domain where I'm having problems uses static IPs. On this problem domain my assigned static IP is in a different subnet (255.255.255.128) than the computers I'm trying to connect to (255.255.255.192). Could this be the problem?

UPDATE: If I ping my machine by name on the network where I am having problems, it resolves my name to an IPv6 address rather than IPv4. As far as I can tell, all the other computers on the network resolve to IPv4 addresses. Is it possible that my computer ended up in a name server with an IPv6 address and that is causing my problems? If so, is there any way to force the name server to store my IPv4 address instead?

I solved this problem today on my LAN by disabling the "Use Sharing Assistant" option on Windows Explorer.

Steps (run on the Windows-7 client machine):

1. Open Windows Explorer;
2. Click the "alt" button to open the context menu;
3. Click on "tools";
4. Click on "folder options";
5. Click on "Exibition Modes" tab;
6. On "Advanced Configuration", the last item of the list may be "Use Sharing Assistant (recommended).

Disabling this item solved my network sharing between XP and 7. I also use static IP's on my LAN.

My setup:

1. Server: Windows XP 32, static IP, Gigabit Ethernet;
2. Client 1: Windows 7 64, static IP, Gigabit Ethernet;
3. Client 2: Windows 7 32, DHCP, 100Mbps Ethernet or 811.G Wifi;
4. Client 3: Same as client 1;
5. Client 4: XBOX 360;

All of the clients are able to see the server with the config above.

EDIT: I also disabled IPv6 on all Windows 7 machines. Not sure if that could help at all.