Every week, someone writes for advice
I just got a new router, and now I can't access my computers from each other.
I just got a new router, and none of my computers can access the Internet.
Frequently, the cause of these problems will fall in two categories - new features, and settings.
Many new, and high end, routers come with protection that emphasise Internet access, and make file sharing an optional activity, to protect the individual computers from each other. Look carefully in the Owner's Manual, for a "DMZ", "Isolation Mode", "Virtual Server", or "VLAN" setting - either on a single port, or affecting the entire LAN.
And if you are setting up a WiFi router, make sure that the radio is turned on. Some WiFi equipment is delivered with the radio turned off, to ensure that you will intentionally activate it, and be prepared for when this is done.
Any time that I was changing my network equipment, I would take a snapshot of all network settings from all computers. You can never tell when this might be a life saver. Logs from "browstat status", "ipconfig /all", "net config server", and "net config workstation" could all be useful when troubleshooting. Make a set before, and after, any change. Compare each, line by line, and if you spot any differences, explain or fix them before continuing.
If you're having trouble accessing the Internet, check to see if your computers are using manual assignments. New equipment will probably include an IP address change for the router - some vendors provide a default LAN on 192.168.0.0/24, others 192.168.1.0/24, and others have additional variations. Maybe the router handles DNS differently too.
If you're having trouble with Windows Networking, either an Error 5 aka "Access Denied" or an Error 53 aka "Name not Found" may be seen, or you may simply not see any computers in Network Neighbourhood. If this is the problem, check the security components on the computers - since a new router will probably result in a new subnet address, check personal firewalls and anti-worm programs, for settings that are IP address sensitive.
If All Else Fails
Get into Troubleshooting Internet Connectivity, or into Troubleshooting Network Neighbourhood, depending upon the problem being observed.
And of course, make sure that the new router has the current firmware, obtained from the vendor.