You seem to have magically connected notebooks

1. Did You invoked "java -jar jedit.jar" at a command prompt? If so, have You had jEdit's program directory as the current working directory? Can You see any output? Is there a jEdit splash screen?

2. When You install jEdit into a new directory, nothing will be deleted, believe it!

3. But jEdit uses always the same settings directory unless You specify the command line option "-settings=dir" (see http://www.jedit.org/users-guide/cli-usage.html#id2496548 ). Using this option You can safely use different versions of jEdit at the same time.

B) If You've installed Your plugins into Your settings directory (which is the default, I think), You cannot use different jEdit versions with this settings directory and propably You can't get back to 4.2, because the old plugins may be overridden meanwhile and Plugin Central isn't maintained for 4.2 anymore, I think.

C) A clean install is simply to use the installer, but:
C-1) It requires a clean un-install which should be possible via system control->software. After that You should re-start Windows and ensure that no jEdit server isn't running in background. Check if there is any "javaw.exe" process in task manager. Then delete the settings directory (.jedit)
C-2) At install, I would try to chose a program directory outside of C:\program files because of some special behavior in Windows 7...
C-3) At install, chose NOT to start jEdit server at Windows startup.
C-4) Install all plugins via plugin manager.

Robert

i have exactly the same problem. the last application i've "installed" was jflap. jflap works fine, but i can't start jedit. i've uninstalled and installed jedit again, but it didn't help.