Sony CLIE PEG-TH55 Handbook page 206

Locking your CLIÉ handheld with a password (Data protection)
5
Tap [Lock & Turn Off...].
The System Lockout screen is displayed.
6
Tap [Off & Lock].
Your CLIÉ handheld turns off and locks.
To turn on your CLIÉ handheld, you must enter the correct password.
Protecting your data with a password
206