The Java Runtime Environment (JRE) does not include 'javac'. The JRE allows you to run java programs, but not compile them. 'javac' is in the JDK.
The JDK was previously referred to as the Java SDK. Maybe you have a j2sdk1.4.2_05 directory. If you do, 'javac' will be in the bin subdirectory. You could always just do a search for 'javac.exe' on your hard drive.