If you changed your computer and want “your PL/SQL developer login history and preferences” as well in your new computer. Copy the following folder to your new computer in the same path format!
C:\Users\[username]\AppData\Roaming\PLSQL Developer
If you need only the login history, open the following file:
C:\Users\[username]\AppData\Roaming\PLSQL Developer\Preferences\[username]\user.prefs
In the file, copy [LogonHistory] part to your new user.prefs file. It’s encrypted but it does not matter. PL/SQL Developer will dencrypt it 🙂
I need this, because I deal with a lots of database with a lots of schema.. It is really hard to remember the passwords sometimes, so i let the PL/Developer to remember them 🙂 (Preferences > Login History > Store with Password)
Akdora's Blog 
What if your old PC was Windows XP?
– Bob
search for user.prefs file in your application setting folders then apply the same trick
Found it in XP under C:\Program Files\PLSQL Developer\Preferences\
[…] PL/SQL Developer User Preferences (Windows 7) | … – Mar 30, 2011 · If you changed your computer and want “your PL/SQL developer login history and preferences” as well in your new computer. Copy the following folder to …… […]
Thank you!
Complementing the information, in the same folder, there is a .ini file (here is default.ini) that saves some other useful informations, like highlight colors, fonts and key configuration