I ordered a pair of new PC's for a dentist office. They have some old software on the Win2K PC, and we need to get it onto the Vista machine. Also, the Win2K machine will stay on as part of the (three PC total) network. The main issue I'm having is that one of the Vista PC's won't communicate with the 2K PC (I haven't tried the other one yet). If I remember, the two Vista's worked fine together. Here's what I've got so far.
Both can access the Internet (DSL).
2K can ping Vista
Vista cannot ping 2K
\\Hostname doesn't work.
The Vista firewall appeared to be off.
If you can't ping the W2K hostname, can you ping the IP? If so, it's a simple DNS resolve issue on your router (or DSL router/modem). Once you can get to the box, you will have to set permissions on the Vista PCs to allow the W2K PC in because Vista uses much tighter perms than W2K. If the data you are sharing are not sensitive, I would just have W2K use Vista's Public shares for data transfers because it makes it much easier to access.
IPV6 uses its own environment, so as long as IPV4 is active on both boxes it should not interfere. If you cannot ping W2K, this sounds like a fw port/DMZ block (router?) or something not quite right on the W2K network settings (especially with both Vistas able to talk to each other). Use IPCONFIG /all on the W2K PC, make sure subnet/DNS/gateway is configured like the Vistas (same network group/Domain, etc., as well). I'm thinking you may find the issue residing in those settings.