I'm at my brother-in-law's place and had to troubleshoot a network issue. I didn't see quite this situation on the web so I'll post it here in case it can help someone else.
The issue was that his laptop (XP) couldn't see his desktop (Vista). The easy guess was that it was a firewall issue but I looked and the Windows firewall was disabled. To throw a wrench in the works, my laptop (Vista) could see the desktop. I surmised then that it couldn't be a firewall issue. A firewall would block both clients, wouldn't it?
After some investigation, I noticed something. When I pinged the desktop from the Vista laptop, the ping address was IPv6. When I did the same from the XP laptop, it was IPv4. I tried the IPv4 address on the Vista machine and it now failed to get a response to the ping. Very strange.
The web didn't appear to be any help. I tried several things none of which helped so I won't enumerate them here. In the end, I found the culprit. It *was* a firewall issue afterall. The desktop happened to have McAfee SecurityCenter (v 7.2) installed which had its own firewall. Disabling that firewall fixed the issue. The data at hand would seem to indiate that this particular firewall only protects the IPv4 stack, however. Oops.
Hopefully this will help at least one of you solve a similar problem.