Which OS do you have? That would help us w/troubleshooting solutions.
The following solutions are for the classic OS systems:

After downloading LW, rebuild your desktop using http://www.versiontracker.com/dyn/mo...]Techtool Lite.

If the above does not work, then try installing LW with extensions off, by restarting while holding down the shift key. This will shut off the extensions.
After LW is installed, then restart your comp to turn extensions back on again.
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If you are using Panther or Jaguar someone else will need to help you as I have no experience using the kitty cat systems.