Locate the LexLib.new file, located in the Toad for Oracle\User Files folder and rename the file to LexLib.lxl.
(Note: This file maybe a hidden file under Toad for Oracle folder.)
Please try a Custom un-install, in many case, this will help remove any corrupted files left behind after a regular un-install.
Here are the instructions to "Custom" un-install.
1. Go to your Toad Start Menu, and click the "Un-install TOAD" shortcut.
2. Use the Option "Custom" un-install.
3. Click the "Select All" button to select all files to be removed, then click "Next" button.
4. Several more similar windows will appear. Do the same on all of them.
5. Continue until you reach "Finish", and then click it.
6. Verify that there are no Toad Folders and Shortcut items left in your computer (perform a Search/Find on Toad).
Delete them if there is any.
7. Restart your computer if it's okay to do so (recommended)
Now try the installation again.