Yesterday I started my iMac G5 and after about 15 minutes, it froze. It's happened before but I can always move the mouse. This is the first time it has actually properly froze. After about 2 minutes, I got a very loud whirring noise. I left it for about 5 minutes and it didn't die down. It has now been left for about 24 hours with the occasional turn on to see if it's fixed. No joy. Now it won't even turn on. I'm sorry I can't give you any specs because I can't open it to get them and I haven't memorised them. PLEASE HELP!