When the screen comes up, try using the "escape" key to cancel it. If that doesn't work, you can try http://baghira.han.de/~jum/LimeWire.html for a version of LW for OSX that doesn't use the the usual installer (If it works, you really owe it to LW to get LW Pro).

If that doesn't work--time to look at your system set-up for old versions of LW and out-of-date system software. I'm guessing you are up to date with OS 10.2.6 and LW 3.4.7, and know how to uninstall the older versions and preferences.

Cheers