Ok, I haven't done any java dev in Windows, but let's see what we can do
Firstly, open a command line and navigate to your Java install directory, e.g.
C:\some\path\to\Java_SDK_1.4.2\bin. It will be different for your system, adjust accordingly.
Make sure that you can run
java from that directory. It should give you the verision info as well as some command-line options. If so, great.
Now, to make things run smoothly from any location on your computer, add the path you just went to (
C:\some\path\to\Java_SDK_1.4.2\bin) to the Windows
PATH environment variable. You can change that from
System Properties->Advanced Tab->Environment Variables (persistant) or using the
set command (temporary). Remeber to simply add it to the path list, not replace it.
With this,
java and
javac should be able to be run from any directory. Check this now.
If all's well at this point, you might want to set some additional envirnoment variables, namely
CLASSPATH (the directory where your code lives) and
JAVA_HOME (the directory where java's installed,
C:\some\path\to\Java_SDK_1.4.2).
Here is a more complete
guide.