I had a similar problem, that was solved by turning off norton's firewall. In my case my old PC would respond to pings, but the new vista PC (an acer desktop) would not. There is a dos program called ping that tests if your cabling and cards and even basic software will respond to LAN (or internet) messages. You need to know the IP adresses of both your PCs, which can be found out by running another dos program called ipConfig. When you know their ip addresses, you can use ping to see if the other PC responds. In my case the new vista PC would not respond, but the old PC would - this meant the cabling and hardware was ok in both directions but some software on the new pc was preventing its response. that software turned out to be norton firewall.

On the other hand, I also have the same problem with sharing that you mentioned, and norton firewall aint involved. So my above cure may not help you.