Here is a check list for you to run through, I know you've probably done most of it, but it never hurts to double check. Even the Windows firewall can sometimes cause this, if you've done anything to change it, including the dreaded clicking the "Restore Defaults" link. If you need any help with any of these post back.
Click start and type services.msc in the search box
Or hold down the windows logo key and hit the R key and type services.msc in the run dialog box
Make sure the following services are running and set to automatic on all machines.
1. Computer Browser
2. DHCP Client unless you are manually assigning static ip addresses to your machine
3. DNS Client unless you have manually assigned static DNS server addresses on you machine
4. Server
5. TCP/IP Netbios helper
6. Workstation
7. Look for Bonjour service and temporarily set it to disabled and stop it.
8. Make sure all machines have the same workgroup name "Workgroup" is fine.
9. All network nodes must be unique, make sure there are no machine name conflicts, and as strange as it may sound
make sure there are no missing or conflicting mac addresses.
10. Make sure Network Discovery is turned on, on the Win 7 machines
11. Under the network and sharing center, change “Advanced Sharing Settings” intelligently to suit your needs. Turn on
network discovery, turn on file and printer sharing, turn on pulic folder sharing, turn off password protected sharing,
and at the bottom Use user accounts and passwords to connect to other computers.
12. If your problem is with down level clients like XP and Vista and your Windows 7 machine is in a homegroup, leave it
and disable IPv6 in the properties of your network adapter.
13. Make sure NetBios over TCP/IP is enabled in the properties of IPv4 under the WINS tab on your network adapter.
14. Make sure file and printer sharing is enable on both and that local firewalls are supporting it.
15. Temporarily uninstall any third party firewalls or Internet Security Suites that may include some type of Firewall
applet (you can always reinstall them later) temporarily disconnect your home network from the internet if this steps
makes you say Whoa!!
16. Clear DNS cache at a command prompt type ipconfig /flushdns
17. Clear netbios name table cache at a command prompt type nbtstat -R
18. Use the ping command, ping each machine from the other by IP address as well as netbios name if either of these fail
you may have skipped over step #15 or the embedded windows firewall may have a problem so turn it off too,
temporarily for testing.
19. Then examine the netbios name table cache at a command prompt type nbtstat -c make sure that each netbios name
is identified correctly by it's proper IP address
20. Disable any network adapters that are not in use
21. Make sure that you have the most current up to date drivers for your network adapter(s) from the hardware device
manufacturer's website
22. Make sure you have the latest firmware updates for your router from the router manufacturer's website
23. In the case of an addon card insure your adapter is seated properly and is identified properly in device manager
24. In the case of a hardwired cable connection inspect the diagnostic lights at both ends of the cable usually amber or
green, steady when connected and flashing when activity is detected, yours may very, don't hesitate to swap out a
suspect cable or swap to a known good port on the router / switch.
