1. Log into your switch, and enter the privileged EXEC mode by entering enable plus the enable password.
2. Enter clear config all to reset the entire system.

You don't need to reload the switch because processing the command wipes the switch. If you've set a boot option, you need to change that option using the set boot command.

If your switch runs Cisco IOS, it maintains a running configuration file and a startup configuration file, both of which you need to clear. Follow these steps:

1. Log into your switch, and enter the privileged EXEC mode by entering enable plus the enable password.
2. Enter write erase, which erases the NVRAM file system and removes all files. At the prompt, confirm that you want to erase all files.
3. Enter reload, and enter no when prompted to save the configuration. (Otherwise, the switch will reload the current running configuration.)
4. Confirm that you want to reload the switch, and your switch configuration is almost clean.

It's almost clean, but not quite. Most people forget to clear any VLAN information they've created for their switches. Depending on the hardware version of your switch and the software version of your OS, the command for this varies. For more information, check out Cisco's “Resetting Catalyst Switches to Factory Defaults” documentation, which walks you through the commands for clearing VLAN information from your switch.