Try these in order shown below.

Hold down keys in the first column and press the key(s) in the second column
| Hold Down: | and press... | what this does: | |
|
|
Command and Option | during startup | rebuilds the Desktop file |
|
|
Command and Option | P and R during startup | resests the PRAM* |
* After resetting your parameter RAM, don't forget to check your date and time, networking, memory and other control panel settings.