It depends on a few things. 1) Which operating system are you using? 2)
How did you install the JVMs in the first place?

I'm going to guess that you are using Windows. I'm going to guess you are
not fully aware of how the JVMs where installed. If my assumptions are
wrong then disregard this message.

IMPORTANT: deleting files from your computer could make your computer
malfunction. I take no responsibility for any damage that might occur to
your computer. This should remove all Java from your computer.

NOTE: if some software installed its own JRE doing this might disable that
application. For example, if I installed WebLogic Application Server I
will find it installed a Sun and JRockit JDK. I never remove them.

- Find all occurances of jvm.dll, java.dll and java.exe.
- Note the directory path, e.g. C:\Program Files\Java\jre1.5.0_03. Ignore
 files in the system directories, e.g. C:\WINNT\ and C:\WINNT\system32.
- Go to Add/Remove Programs and remove all occurances of Java.
- Edit your environment variables. Remove all reference to Java in the
 PATH and remove the CLASSPATH.
- Go to the directories containing Java binaries, as noted above, and
 delete all the files and folders. EXCEPTION: do not delete the system
 directories.
- Go to C:\Documents and Settings\<login>\Application Data\Sun, where
 <login> is your Windows login, and delete the Java folder
- Go to C:\WINNT and delete the Java folder
- Go to C:\WINNT\system32, find and delete all files starting with Java.
- Go to the Start menu and using Run... run regedit.exe. NOTE: deleting
 things in regedit.exe gives no ability to undelete. It does not use the
 trashcan. Be VERY careful in there.
- Use Find... to find all occurances of the directory paths noted above.
 For example, if you had a JDK installed in C:\jdk_1.4.2_08 then search
 for C:\jdk_1.4.2_08.
- Remove the relevent data. Sometimes it will be what you found. Sometimes
 it will be the parent folder. You'll have to figure that out on your
 own.
- Reboot your computer.

I have never cleaned someone's computer without finding some exception to
the above set of instructions. They server as a guide but generally I have
to delete more or less than what I have described.

Good luck.

Go into the control panel and select add/remove software.

Then when done delete all files in the corresponding JRE and JDK
directories.

If you really want to be thorough, remove some registry entries as
well see http://mindprod.com/jgloss/registry.html

When you install your new JRE test it. See
http://mindprod.com/jgloss/jre.html#TESTING