Note: These instructions are for the Treo 600. For other models, see the External References section below.

Soft Reset
What it does: A soft reset is similar to restarting a computer. No information is deleted.
How to do it : Use the reset tool to gently press the reset button on the back of your Treo.

System Reset
What it does: A system reset, also called a safe or warm reset, tells your Treo to stop what it’s doing and start over again without loading any system extras.
How to do it: While holding Up on the Five-Way navigation pad , use the reset tool to gently press the reset button on the back of your Treo. When the Palm Powered logo appears, release Up.

Hard Reset
What it does: A hard reset erases all data and third-party software on your Treo.
How to do it: While holding the Wireless Mode button, gently press the reset button on the back of your Treo. When the Palm Powered logo appears, release the Wireless Mode button. When the warning appears, press Up on the Five-Way navigation pad to confirm the hard reset.

Zero Reset
A zero out reset will regress a device that has non-volatile memory to its factory-fresh state. When done correctly, it completely rewrites your device's internal memory with zeros and ones, ensuring that any data is expunged. It should be used only if you want all information completely obliterated (example: you're selling your handheld, or you're sending it in for repair, and want to protect your privacy).

1. Read through these instructions before attempting the reset. We made this method of zero out reset extremely awkward to perform, so that it would not happen by accident. You may need the help of a dextrous friend if you find it too difficult to do by yourself.
2. Connect your device to its HotSync cable or cradle. The HotSync cable does not need to be connected to your PC, and it does not need to be connected to power.
3. Press and hold the Power button and UP on the 5-way navigator.
4. While continuing to hold Power and UP, press and hold the HotSync button on the HotSync cable or cradle. As you press HotSync, make sure your other finger doesn't slide to LEFT or RIGHT on the 5-way navigator; it needs to be exactly on UP during the entire process. Although you are pressing the HotSync button, a HotSync operation should not begin.
5. While continuing to hold Power, UP and HotSync, press and release the RESET button on the back panel of your device . This is very difficult to do with only one person; you may wish to hold the stylus in your mouth and use your hands to press Power, UP and HotSync.
6. Release Power, UP and HotSync.
7. If you did this reset correctly, the screen of your device will go blank, and you will not be able to turn it on. The charging LED will not light up, even if it's connected to power. It will appear to be "dead." (any other activity — such as the Palm OS logo or a rainbow-colored Boot Log screen appearing — means the reset was performed incorrectly; try again)
8. Your device will appear "dead" for several minutes (up to 10 minutes). During this time, your device's internal memory is being reformatted. If your device doesn't appear "dead" for several minutes with the screen completely blank, the zero out reset was performed incorrectly; try again.
9. After several minutes, your device will "wake up" and the Palm and Palm Powered logos will appear as if you had performed a hard reset. Eventually, you'll be taken to a series of screens to calibrate the touchscreen and set date & time.

External References
Palm Knowledge Library instructions for Resetting your device (all Treo devices). If the link is broken, search for "soft, system/warm" (with quotes).

Separate instructions by model number on mytreo.net:

Resetting your Treo 600
Resetting the Treo 650