Go to your admin pannel and select "uninstall programs".
Scrool down to letter "J" and uninstall any version of Java you can see there.
Then, go to
http://www.oracle.com/technetwork/java/javase/downloads/index.html and click the "Download" button below "JRE". If you are a Java developper, click the "Download" button below "JDK".
Tick "Accept Liscence Aggreements" and select the version of Java for your computer. If you are running Windows x64, choose the ".exe" file instead of the ".tar.gz" one which is a bit more difficult to install.
Run the file right after you downloaded it and install Java.