So I have two Cisco 800 routers (one on my desk at work see picture) and one in my lab at home. I want to build an IPSEC VPN tunnel between them just to show a tunnel working. To do this, I first need to wipe the routers of any existing config. I did this;

I consoled into it, (using PuTTY) (NB: hostname has been set to cisco800atwork)

• Type enable to enter the correct mode.
• Type configure terminal. The cisco800atwork(config)# prompt   appears.

• cisco800atwork(config)#config-register 0x2142 (this causes the router to ignore the current config when it 
  reboots)
  cisco800atwork(config)#end
  cisco800atwork(config)#reload (this causes the router to reboot)
  

• The router will ask if you want to save the configuration, I said NO to this. I then confirmed the reload at the following prompt. I then answered NO to using the configuration dialogue once it had rebooted.

• cisco800atwork(config)#config-register 0x2102 
  cisco800atwork(config)#end 
  cisco800atwork(config)#write memory (writes this blank configuration to memory)
  cisco800atwork(config)#reload
  
  

Now there is a quicker way of doing it, using the write-erase command, which looks like this;

• cisco800atwork(config)#config-register 0x2102 
  cisco800atwork(config)#end 
  cisco800atwork(config)#write-erase
  cisco800atwork(config)#reload
  

Oh, its a good idea when borrowing, sending and receiving Cisco equipment, just like any other IT hardware, that you get into regular habit of wiping them. So these commands are good to know.