This answer was taken from here. The person seemed to experience the same problem under OS X Mountain Lion (10.8.3). Maybe these steps will do the trick for you, too.
Please read this whole message before doing anything.
This procedure is a diagnostic test. It’s unlikely to solve your problem. Don’t be disappointed when you find that nothing has changed after you complete it.
The purpose of the test is to determine whether the problem is caused by third-party software that loads automatically at startup or login, or by a peripheral device.
Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards. Boot in safe mode and log in to the account with the problem. Note: If FileVault is enabled, or if a firmware password is set, or if the boot volume is a software RAID, you can’t do this. Ask for further instructions.
Safe mode is much slower to boot and run than normal, and some things won’t work at all, including Wi-Fi on certain iMacs. The next normal boot may also be somewhat slow.
The login screen appears even if you usually log in automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.
Test while in safe mode. Same problem?
After testing, reboot as usual (i.e., not in safe mode) and verify that you still have the problem. Post the results of the test.
Another person who experienced a more complex problem incl yours. The entire thread can be found here. But the problem was solved like this:
- I downloaded Mountain Lion from Apple Store (yesterday).
- Today I executed the Installation File form Launchpad.
- The installation executed perfectly and now everything is working again.
- Everything: Missing Icons, Spotlight ... is now perfect!
Maybe this helps, too.