Could be for a no. of reasons
Have you checked if Nvidia have updates for your drivers?
Could be a ram issue if you have less than 500 MB. Good idea to defragment your HDD if you can get around to it also!!! Strange things can start happening otherwise.
Make sure your java is at least up to version: Java 1.4.2 (click on link) LW will only ask for Java 1.3 when it installs but really 'needs' a later version.