Htc p3400i hw to format window & reinstall it

Try a Hard Reset
Note: this will delete all data, so mack sure you have backedup anything you need to keep (ie addressbook, personal files)
Press and hold the left SOFT KEY and the right SOFT KEY, and at the same
time, use the stylus to press the RESET button at the middle left side of your device.
Release the stylus, but continue pressing the two SOFT KEYs until you see the following message on the screen:
This operation will delete all your personal data, and reset
all settings to manufacturer default. Press Send to restore
manufacturer default, or press other keys to cancel.
Now Release the two SOFT KEYs, and then press the TALK (Send) button

(Info from: http://www.efsoft.it/resetdev.htm)