You may have an incompatible startup item in your user account. You may need to start the computer in Safe Mode and disable login items by using the following steps:
- 

- Shut down your computer. 

- Wait 30 seconds 

- Hold down the Shift key after pressing the power button. 

- When you see the gray Apple logo and progress indicator (looks like a spinning gear), release the Shift key. 

- If you are prompted to log in, type your password, then hold the Shift key again when you click Log in. 

- Choose Apple () menu > System Preferences. 

- Click Users & Groups. 

- Click your account name, then click Login Items. 

- Look for any application that has a prohibitory sign (a gray circle with a slash) over its icon. This indicates an incompatible item. 

- Select the incompatible login item, then click the Delete (-) button. 

- Restart your Mac. 

Learn more
If you receive an alert about any software being moved to a folder called "Incompatible Software" after upgrading to OS X Mountain Lion, please check with the software vendor to see if they have released a Mountain Lion-compatible version.