Well I tell you an oddity, on my intel macpro I am unable to run 4.18 or LW 5. So I installed OSX afresh onto a different drive & 4.18 & LW 5 worked fine. (BTW I had reinstalled OSX a few times over past 12 months on the main drive but only archive installs, not reformatting the drive.)

Setting up a test account was just an idea. You can always delete the account anytime. Thing is if LW does work fine there, then it points to a problem in the main account rather than the computer itself.

There might be a problem with your Java installation. Perhaps one of the installations was partially corrupt or related issues. Perhaps Java preferences corruption. Sometimes solutions to LW problems on OSX was reinstalling Java using the offline installer, as compared to software update. Or even the OSX version itself.

BTW you upgraded to Leopard right? What type of installation did you do, an archive instal or ... ?