There's a prx file that when you log into your psp (turn it on) it asks for a password.
The only issue is that it can be "overwrittern" if you just pull the memory card out. I used to use it when i lived in the dorms in college, cuz roommates liked to borrow my stuff. PM if you want it. I'll mediafire it for u.
Works on5.00 and 5.50
Wynd