Have you got a Cisco 1841 (or 1800 series) router and forgot user name and password or just need to reset it to its factory settings. Follow these simple steps to reset your router to factory defaults.
(I have also found this to also work with many other models of Cisco routers)
Connect you PC to the router via console cable and connect using Putty.
Boot the router while holding ctrl and break get into rommon1>
Type the following command
Type Reset to start the factory reset
The router reboots, but ignores the saved configuration.
To enter config mode, issue the config terminal command. You should now see a Router(config)# prompt.
To change the configure register to recognize the startup configuration, issue the config-register command, as shown in this example:
Router (config)# config-register 0x2102
Exit from configuration mode, press Ctrl and Z.
To save the blank configuration, issue the copy-running config-startup config command.