Probably won't be much help, I'm not an expert and I'm not a Vista user but...

I assume that both PCs are connected to your router. Do either of the PCs report any problems with the network connection? Are they both set to obtain IP addresses automatically? The router should then allocate them an IP address and act as DHCP and DNS server.

Are both PCs part of the same WORKGROUP or DOMAIN?
Have you enabled file sharing on the tv-pc? And shared some folders on there.
Do you have any firewall software on the tv-pc that might be preventing your main PC from connecting to it?

steve