iMac 2009 frozen after booting and startup

So, a few days ago, my iMac got stuck in the white loading screen that everybody says. It happened before already, and at the time, I just booted holding the key C and installed the OS with the CD. That went ok, nothing wrong. But then, a few days ago as I said, it happened again. At first, I tried booting it with the keys Command + Option + P + R to start with the OS utilites. I tried several different keys combinations and nothing worked but the C. So I booted it again with the C key. I installed the OSX Leopard again but now, when I turn the iMac on, it will go to the desktop screen just fine BUT it is frozen still. The mouse cursor is at the loading colourful ball and I can't use nothing, it just stays there forever! Like, I can't even click at the Macintosh HD folder or the at the Finder in the menu. It is really frozen at the loading cursor ball and it stays there forever.

Does someone know at least what is causing this?

How can I solve this problem without going to Apple Care? I do not have the money to leave it there, since the Apple Care in Brazil it's expensive.

Please help! I would be forever glad!

PS: If there is any doubt, this is the loading cursor I was talking about, and it's driving me insane!!!