I assume you've got WIRED connections into the SWITCH ports on your router to both PCs.
I also assume you have basically a dumb unfilltered SWITCH on the router and not a managed / firewalled one.
Many wireless routers and even some wired ones have some kinds of security options that prohibit clients from talking to each other or which can institute firewall rules even between local clients.
*) make sure both PCs have different MAC addresses, e.g. you're not overriding them to be the same number or whatever.
*) Make sure the "network type" of your network is listed as HOME or BUSINESS network. If it says that you're connected to a PUBLIC network, file sharing and discoverability will be off by default.
...[If you are trying to have them in DOMAIN mode instead then you better make sure you have a functional DOMAIN controller / WINS / DNS server they can both contact etc. etc. if you want networking to work.]
*) Make sure autonegotiation is functional and working for your NICs and the switch, e.g. they're both ending up at 100MB full duplex or whatever makes sense for your hardware. Sometimes there are incompatibilities with autonegotiation and you lose data.
*) Make sure "client for microsoft networks", "file and printer sharing", "IPV6" and the two options for "link layer topology discovery" the responder and the I/O driver are ALL enabled on the adapter properties of each PC's NIC. It uses LLTD to help find other Vista machines on your network.
*) Make sure both machines are in WORKGROUP networking mode, that the WORKGROUP name is identical between them, and that the COMPUTER NAME for each one is set to something unique.
*) Maybe you should try installing a firewall like Comodo if only so you can EXPLICITLY view / enable the sharing rules rather than assume that the firewall is totally "off" and thus permitting any traffic. Specifically:
UDP IN/OUT FROM ANY PORT TO PORTS 137,138
TCP IN/OUT FROM ANY PORT TO PORTS 139,445
are the "allow" rules that you should have set for any IP address in the network subnet that your router / LAN is on.
*) Make sure you're getting unique IP addresses assigned to each PC, e.g. over DHCP from your router or whatever you're doing. Identical IPs = bad. IPs not from the same subnet = bad. Netmasks not allowing the router and both PCs and your gateway and broadcast address to be within the subnet = bad. IP conflict with your gateway = bad. IP conflict with router = bad. IP confict with broadcast address = bad. IP conflict with DNS servers = bad.
*) Enable simple file sharing on each PC and allow the PC to be discoverable to others and everything should work.
*) When in doubt, reboot, sometimes it helps to get networking going when the machines get confused.
*) Try Map Network Drive to \\192.168.1.5\Sharename or whatever even if the network neighborhood discovery doesn't list the other PC. Obviously you have to give it the right IP address, share name, and user credentials when doing this. Dropping to an administrator command prompt and doing this through "NET USE" commands can help too.
*) IPCONFIG /ALL from the admin. command prompt look reasonable on each machine after they both have been booted and networked for a couple of minutes (e.g. DHCP in place etc.)?
