IMAC starting veryyyyy slow....

Question

I have a 21,5" iMAC mid 2010, and I noticed that it took more and more time every time I tried to start the computer when I had the Snow Leopard SO. At that time it reached up to 20 to 30 minutes to start from the moment I pushed the power on button, until the desk was available.
The I moved to Mountain Lion SO when it was released, and the problem seemed to disappear, since it began to start quickly again, as in the beginning.
But unfortunately the problem is coming again, because it began to increase the starting time every time, and now it is quite worse than before, because last time that I intended to start it took ¡nearly 2 hours! At the end it finally starts... Once the computer is in operation I do not notice anything wrong in normal operation but it is clear there is something wrong. I normally leave the iMAC in stand by (so I do not have to restart every time) and in this way I can get inmediately my desk by pressing any key and I don't need to wait so long.
I tried to clean up the computer with apps like Magican or CCleaner, but it didn't work. I did'n find anything wrong apparently in the initial programs, SO, etc...
Please, HELP....What can I do to make the start time quicker as it should be?
Thanks in advance,
Alejandro

Answer 1

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. The instructions provided by Apple are as follows:
Shut down your computer, wait 30 seconds, and then hold down the shift key while pressing the power button.
When you see the gray Apple logo, release the shift key.
If you are prompted to log in, type your password, and then hold down the shift key again as you click  Log in.
*Note: If FileVault is enabled under OS X 10.7 or later, or if a firmware password is set, or if the boot volume is a software RAID, you can’t boot in safe mode. Safe mode is much slower to boot and run than normal, and some things won’t work at all, including wireless networking on certain Macs. 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. Startup will be slow, but it shouldn't take two hours. 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.