If you notice a problem with Lightroom the first time you start it, something may have gone wrong during the installation process. Uninstalling and then reinstalling may be all that's required to correct the problem. The process is different for each operating system, so here are the separated steps for each.
On Windows:
Close all running applications.
Choose Start→Control Panel→Add or Remove Programs.
Select Lightroom from the applications list and click Remove.
Close the Add or Remove Programs dialog box.
On a Mac:
Open the Applications folder and drag the Lightroom application to the Trash.
(Optional) Open the OS X Library folder (not the Library folder under the User account), open the Receipts folder, and send the Adobe Photoshop Lightroom.pkg file to the Trash.
This optional Step 2 should be used only if you're downgrading to an earlier version of Lightroom or you're having trouble installing a new version. Otherwise, you can skip this step.
Uninstalling Lightroom doesn't touch the catalog file, the preference file, your presets, or your photos. Uninstalling Lightroom removes only the application. When you reinstall, you find all those other files just as you left them. To reinstall on either platform, just double-click the installation file and follow the installation instructions.