Factory Reset a Cisco ASA5505

Here's how I did it:

Connect Putty (or equivalent) to the console and boot the unit

As it boots you will see something like the entry below. 
Press ESC when you see this:

Use BREAK or ESC to interrupt boot.

You'll be taken to the rommon #0> prompt

enter

confreg 0x41

Update Config Register (0x41) in NVRAM...

confreg

You'll see the following show up with some questions

Current Configuration Register: 0x00000041

Do you wish to change this configuration? y/n [n]: Y

enable boot to ROMMON prompt? y/n [n]: Y

select specific Flash image index? y/n [n]: Y

  enter Flash image index [1->7]: 1

disable system configuration? y/n [n]: Y

go to ROMMON prompt if netboot fails? y/n [n]: Y

enable passing NVRAM file specs in auto-boot mode? y/n [n]: Y

disable display of BREAK or ESC key prompt during auto-boot? y/n [n]: Y

You'll come up to a ROMON prompt

BOOT

once unit is booted 

ciscoasa> enable

no password, just press enter

ciscoasa# copy startup-config running-config

ciscoasa# configure terminal

Would you like to enable anonymous error reporting to help improve

the product? [Y]es, [N]o, [A]sk later: N

ciscoasa(config)# password password
ciscoasa(config)# enable password password
ciscoasa(config)# username cisco password password
ciscoasa(config)# no config-register
ciscoasa(config)# copy running-config startup-config
ciscoasa(config)# config factory-default

Factory-default configuration is completed

PowerCycle the unit and you'll be taken to the 'ciscoasa' prompt and can proceed with regular configurations.