Uninstall LW & ZA (don't uninstall LW via add/remove programs, go start > all progs > LW & use the uninstall option from there).
Now, reinstall LW. Once that's installed, reinstall ZA. Reinstalling LW before ZA has been a fix for a few people

If that doesn't help, try this version of Java. Download the JRE offline installer & remove any other Java versions before you install this one.
Archive: Download Java 2 Platform Standard Edition (J2SE) 5.0 Update 11

When you set up port forwarding, did you set up a static IP address first? You'll need to, or port forwarding won't work. Did you use these instructions?
Port Forwarding for the Linksys WRT54G
LW's listening port, port in the 'manual port forward' box & port forwarded within the router should all be the same number.