I'll elaborate a little on Ethel's answer and then I'll tell you what to do when it doesn't work!
As Ethel states, the first thing to do is to check your browser's settings. In Internet Explorer 6, the route is
Tools > Internet Options > Advanced.
Check to see that, under 'Java (Sun)', there's a tick alongside 'Use JRE . . ."
The instructions for Internet Explorer 7 are probably very similar to those for IE6.
With Firefox the route is
Tools > Options > Content.
Check that there are ticks alongside both 'Enable Java' and 'Enable Javascript'.
However, I suspect that you'll find that the relevant ticks are already in place. If so, go to Add/Remove programs and remove Java (which will appear in the program list as something like 'J2SE Runtime Environment'). Then download and reinstall Java, from here:
http://www.java.com/en/download/index.jsp
Chris