It's my guess the problem is similar to my system. I am unable to run LW 4.18 or LW 5 on my main OSX system. Despite doing a few OSX archive re-installs last year, that did not solve the problem. I used my old bootcamp partition for windows & reformatted it. Installed OSX onto it. Installed LW 4.18 & it worked fine. Installed LW 5 when it came out, worked fine ever since. So something was wrong with my original OSX installation. So last year I should probably have done an "Erase and Install" instead on my main OSX. I just didn't wish to have to go thru the pain of backups, reinstalling all softwares again.

Perhaps that is not your problem. Set up another account, a test account: Setting up a Test Account in OSX (click on link). Try opening LW from there. Also try SAFE Boot & Repair Permissions.

You could try reinstalling OSX using the combo updates; Apple - Downloads - Mac OS X - Mac OS X Updates or alternatively a java update: Apple - Downloads - Mac OS X - Application Updates. This used to be a fix for OSX 10.4 & earlier, but not sure if it works for Leopard. There seems to be no OSX or Java receipt files either. I just tried the last Java update, seemed to install ok. BTW should normally repair permissions after installing any software, especially system software such as OSX updates or Java & some other 3rd party program updates.