If it is not possible to set the JDEV_USER_HOME env variable (because Admin rights are not granted for the PC, for example), use a batch file to set the variable, and start JDeveloper from the command line:
set JDEV_USER_HOME=C:\Oracle\JDevR1204\jdevhome\jdev
call C:\Oracle\JDevR1204\jdevbin\jdev\bin\jdevW.exe
Good one. Very usefull to those restricted by admin rights access