these are the command i did to reset a cisco pix 515e firewall:

<firewall> en

Password: *****

you will see it changes to enable mode:

firewall#

firewall# write erase

reboot the firewall
firewall# reload

Erase configuration in flash memory? [confirm] yes

[OK]
firewall# reload
Proceed with reload? [confirm]
firewall#


***
*** --- START GRACEFUL SHUTDOWN ---
Shutting down isakmp
Shutting down File system



***
*** --- SHUTDOWN NOW ---


CISCO SYSTEMS PIX FIREWALL
Embedded BIOS Version 4.3.207 01/02/02 16:12:22.73
Compiled by morlee
256 MB RAM


wait until it boots and you get to this prompt:

Pre-configure Firewall now through interactive prompts [yes]? yes

this is my example:
Firewall Mode [Routed]:Routed
Enable password [<use current password>]:******
Allow password recovery [yes]?yes
Clock (UTC):UTC
Year [2010]:2010
Month [Apr]:
Day [3]3
Time [00:38:12]:16:45:17
Inside IP address: 10.0.0.150
Inside network mask: 255.0.0.0
Host name: firewall
Domain name: examle.com
IP address of host running Device Manager: 10.0.0.151

The following configuration will be used:
Enable password: <current password>
Allow password recovery: yes
Clock (UTC): 00:38:12 Apr 3 2010
Firewall Mode: Routed
Inside IP address: 10.0.0.150
Inside network mask: 255.0.0.0
Host name: firewall
Domain name: example.com
IP address of host running Device Manager: 10.0.0.151

Use this configuration and write to flash? y
INFO: Security level for "inside" set to 100 by default.
WARNING: http server is not yet enabled to allow ASDM access.
Cryptochecksum: ********** ********* ******* **********

1513 bytes copied in 0.350 secs

Done

thats it

NOTE: if you need more help, you can go to the cisco website, it shows you step by step instructions how Restore the Default Configuration

To restore your default configuration back to the factory-default values, enter the following CLI commands by completing the following steps:


http://www.cisco.com/c/en/us/td/docs/security/pix/pix63/quick/guide/63_515qk.html
How To Save Changes Cisco Pix Firewall