Xp & Vista machines wired networking not happening

Would be great if someone knows a simple solution. Seems like nothing is simple. I have hooked up a couple home networks already and know a bit. I have a client that has 2 machines wired into the same router. However, it is a Motorola that connects them. #1 Vista to Motorola, #2 XP Pro to Motorola. They both can access the internet. #1 sees #2 but cannot open it because (here’s the clue) it says it can’t read the dns name of #2. #2 does not see #1 at all. The workgroup and network names are the same. I did this at home between a vista, xp, & W98 with no problems. What is the dns thing? And how do I step by step fix it? Thanks a lot!