Macbook Air login screen twice I have a serious problem with my 2011 MacBook Air (current OS version (Mountain Lion))
Always on startup, I have to enter my password twice.
Right after entering it the first time, the screen is gray for a very short time and the same login screen appears again. After entering the password the second time, it takes me to my desktop as usually...

I also tried activating the automatic login, but then a different problem occures:
The desktop appears (without wallpaper and dock), after less than a second the screen is gray again, the screen appears again (still without wallpaper and dock). After about 10 seconds the dock slides up and the wallpaper appears...

Is it possible to reinstall Mac OS without changing any files and settings?

Any help would be great!
yes its really easy to reinstall it without damaging anything! however a backup is ALWAYS recommended, if you bought it over the Mac Appstore you can just redownload it for free and install it again! or if your computer came with mountain lion you can

Choose Apple menu > Restart, and then hold down the Command (⌘) and R keys while the computer restarts.
If you’re not connected to the Internet, choose a network from the Wi-Fi menu in the right corner of the menu bar (in the top-right corner of the screen).
Select Reinstall Mac OS X, and then click Continue.
Follow the onscreen instructions. In the pane where you select a disk, select your current Mac OS X disk (in most cases, it is the only one available).
To start the installation, click Install.