Ok I don't even remember..
Go through all this though. Hopefully we find it.
Every computer on the network has a name
Every computer on the network is in the same Workgroup, with no spaces before or after
Assign each computer an IP address. Navigate to your router from a web browser [usually 192.168.x.x in the address bar, different for each router] and disable the DHCP Server. Then follow this picture [it's slightly different in Vista but not really.]. Start with 192.168.x.x+1 and go up one for each computer. As an example, if I assumed your router was 192.168.1.1, and you have three computers, each computer would be 192.168.1.2/3/4 http://img359.imageshack.us/img359/4448/halpat1.jpg
All firewalls should be set to accept the IP addresses on your network. Most programs let you set a range of allowed addresses [in my case I allow 192.168.2.1-192.168.2.7]. Include the router and all devices on the router.
On your Vista computer, allow everything I have allowed, and make sure the network type is 'private'. http://img172.imageshack.us/img172/194/lulcn7.jpg
Edit; hopefully you're not extremely computer savvy and already did all these things. I'll try and think of what else could be an issue, hopefully someone can chime in as well if nothing works.