First of all, try upd@ting/reinstalling Java.

Go here (click on link) Java SE Downloads & download the current JRE offline installer.
Next, remove any other Java versions already installed on your computer via Add/Remove Programs or Vista's equivalent.
After that, install the Java that you just downloaded. Restart your computer.

If that doesn't fix things, go through steps 2 - 8 in this thread (click on link): http://www.gnutellaforums.com/333278-post2.html