Q: downloaded osx yosemite but want to go back to mountain lion OS X

If your computer came with Snow Leopard pre-installed:

Clean Install of Snow Leopard

Be sure to make a backup first because the following procedure will erase

the drive and everything on it.

     1. Boot the computer using the Snow Leopard Installer Disc or the Disc 1 that came

         with your computer.  Insert the disc into the optical drive and restart the computer.

         After the chime press and hold down the  "C" key.  Release the key when you see

         a small spinning gear appear below the dark gray Apple logo.

     2. After the installer loads select your language and click on the Continue

         button. When the menu bar appears select Disk Utility from the Utilities menu.

         After DU loads select the hard drive entry from the left side list (mfgr.'s ID and drive

         size.)  Click on the Partition tab in the DU main window.  Set the number of

         partitions to one (1) from the Partitions drop down menu, click on Options button

         and select GUID, click on OK, then set the format type to MacOS Extended

         (Journaled, if supported), then click on the Apply button.

     3. When the formatting has completed quit DU and return to the installer.  Proceed

         with the OS X installation and follow the directions included with the installer.

     4. When the installation has completed your computer will Restart into the Setup

         Assistant. After you finish Setup Assistant will complete the installation after which

         you will be running a fresh install of OS X.  You can now begin the update process

         by opening Software Update and installing all recommended updates to bring your

         installation current.

Download and install Mac OS X 10.6.8 Update Combo v1.1. Download Mountain Lion from your Purchases page in the App Store.

What version of OS X came pre-installed on your computer when it was new? Does it run Snow Leopard or only Lion and later? If the latter, then you can do this:

Install OS X Using Internet Recovery

Be sure you backup your files to an external drive or second internal drive because the following procedure will remove everything from the hard drive.

Boot to the Internet Recovery HD:

Restart the computer and after the chime press and hold down the COMMAND-OPTION- R keys until a globe appears on the screen. Wait patiently - 15-20 minutes - until the Recovery main menu appears.

Partition and Format the hard drive:

1. Select Disk Utility from the main menu and click on the Continue button.
2. After DU loads select your newly installed hard drive (this is the entry with the mfgr.'s ID and size) from the left side list. Click on the Partition tab in the DU main window.
3. Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Click on the Options button, set the partition scheme to GUID then click on the OK button. Set the format type to Mac OS Extended (Journaled.) Click on the Partition button and wait until the process has completed. Quit DU and return to the main menu.

Reinstall OS X: Select Reinstall OS X and click on the Install button. Be sure to select the correct drive to use if you have more than one.

Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.

This should restore the version of OS X originally pre-installed on the computer.

I suggest you download and install EtreCheck from http://etresoft.com/etrecheck

Run EtreCheck and publish report here

Allan