1. Switch to enable mode:

   enable

2. Clear the runtime configuration:

   clear config all

3. Erase the saved configuration:

   write erase

4. When asked for confirmation confirm by pressing the enter key:

   Erase PIX configuration in flash memory? [confirm]

5. Reload the firewall:

   reload

6. When asked for confirmation confirm by pressing the enter key:

   Proceed with reload? [confirm]

7. The firewall should restart.
8. After startup the configuration is reset and the firewall asks to configure it:

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

Sources:

• Cisco Community - How to reset the PIX Firewall to factory default