Open the java control panel and on the Advanced tab, under 'Default Java for Browsers' make sure your browser is listed and ticked.
Check that the CLASSPATH env. variable is set to a valid path (open a command prompt and type SET and Enter to view variables):
CLASSPATH=.;C:\Program Files\Java\jre6\lib\ext\QTJava.zip
Open IE and clock on Tools, Manage ADd ons and make sure the following plugins are enabled:
Java(tm) Plug in 2 SSV Help
JQSIIEStartDectectorImpl Class
Visit this page to test the the plug-in is working with your browser:
http://java.com/en/download/help/testvm.xml