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:OracleJDevR1204jdevhomejdev
call C:OracleJDevR1204jdevbinjdevbinjdevW.exe