Download and save the latest version of Java 6 update 18 (JavaSetup6u18-rv.exe) to your desktop from the page linked below. Remember to save the file not run.
If the installer is not the correct one click on the "See all Java downloads link" and select the correct version:
Before you install the latest version, after you have uninstalled all older versions clear your temporary internet files then reboot. After reboot double click (or right click and run as administrator) JavaSetup6u18-rv.exe to install.
If you still have problems with the Java install and haven't run the Windows Installer Clean Up utility, try running it and searching for Java remnants and try the install again.