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.