Uninstalling then reinstalling Lightroom will not change your Lightroom Preferences since the preference file, and a few other settings and files, are not deleted when you uninstall Lightroom.
To reset Lightroom to its defaults you need to close the application down, then do a search on your C:\ drive for a file named Lightroom 3 Preferences.AGPREFS and delete it.
When you restart Lightroom it will create a new AGPREFS file. The AGPREFS file is stored in the C:\Users\[USER NAME]\AppData\Roaming\Adobe\Lightroom\Preferences file path on a Windows computer. By default the AppData folder is hidden so you'd first have to configure Windows to Show Hidden Folders and Files before you can see the file.
So the steps I'd take is to delete your existing AGPREFS file, restart Lightroom and reconfigure it to your requirements, and it should work without problems.