My IMac won't fully turn on Please Help!!!?

Ok so, I have an IMac right? It's running OS X 10.5 and one day I had to force turn off my Mac because it froze. Then when I wanted to turn it on it went to grey screen with apple logo and spiral tingy at bottom and it jsut stays there. Sometimes if i leave it of for like 4 hours or so it wors perfectly fine until i have to shut it off...I close all apps normally (apple+q) and then when i'm on finder I go to apple thingy to turn it off and the colorful ball appears and It's the loading ball am I right? Ok so it gets to that ball and stays like that and ball won't leave...So to turn it off I have to force turn it off and I once had this problem with a Mac Book Pro which is the one i'm on but I one day left the MacBook on the same screen and it worked perfectly and it never had that problem so I did this with the IMac and if i leave in grey screen for like 30 minuted and it goes to blue screen from there sometimes it continues working like goign from blue to my background to working fine and sometimes it get's stook in bue screen or at background. So i'm asking if you guys know what i can do? I've tried the control+option+r+q thing and it restarts but same problem i'm actually getting scared because we live in Colombia and we can't send to apple or anything and it's my dad's computer so we can't let it break please help me.