The following is from the Ipaq 5550 Manual.
To perform a full reset:
1. Press and hold Application Buttons 1 (Calendar) and 4
(iTask) on the front of the unit and, at the same time, use the
stylus to press the Reset button on the bottom of your iPAQ
2. Hold all buttons until the displayed image begins to fade
(approximately four seconds).
3. Release the Application Buttons and stylus.
?To verify a full reset, press the Power button. If the iPAQ Pocket
PC has been fully reset, it does not power on.
4. Reactivate the battery by:
? Connecting the iPAQ Pocket PC to AC power
? Pressing the Reset button with the stylus