You can add this line to eclipse.ini :
-vm
D:/work/Java/jdk1.6.0_13/bin/javaw.exe <-- change to your JDK actual path
-vmargs <-- needs to be after -vm <path>
But it's worth setting JAVA_HOME and JRE_HOME anyway because it may not work as if the path environment points to a different java version.
Because the next one to complain will be Maven, etc.