To start you can delete safely any music or video file.
If that is not enough to fix the problem, then the best thing to do is resetting the PDA to factory settings after backing up all your personal data.
After backing up your data with HotSync, perform a hard
reset, this will erase all applications and returns your device to its
factory default settings.
To perform a hard reset
1. Hold down the power button on the front panel of the PDA.
While holding down the power button, use a paper clip to press the reset button.
3. Release the power button.
4. You will get a message warning that you are
about to erase all the data stored on the PDA, do the
Press the top scroll button on the front panel
to complete the hard reset and display the startup screen.
If you press any other button you will do a soft reset, that will not fix the problem. In that case restart the procedure.
5. After doing hard reset the unit will return to the initial factory state. All third party installed applications will be deleted, and setting will be reported to factor default.