About 10 days ago I attempted to upgrade my nearly 4 year old G5 Intel from Leopard to Snow Leopard. About a third of the way through I got an error message that said that the install disk was dirty and should be cleaned. I cleaned it several times with a spectacle cloth before going back to the store and getting a replacement. This gave the same error. Worse still, the computer would no longer boot up Leopard, I simply got the grey screen with Apple logo and spinning wheel. I tried to restore from Time Machine - same problem. I came here and received some help and the computer booted and ran fine for a couple of days before hanging again. Sometimes I can get to the point where I register an account, but usually I get variations of grey, blue or black screens. I have done innumerable clean installs from Tiger thru Snow Leopard and Time Machine restores and they all hang and give me an error message: INVALID NODE STRUCTURE Yet when I erase the HD I run verify and it says that it's fine. I am now at my wits end - not least because I'm not sure that there's actually too much wrong with the hardware, it's just that I don't know the buttons to press! Any help will be very much appreciated. My Applecare has expired so I can't call them. Having just bought my Daughter a Macbook Pro 15" (hence I can send this message) I'd like to fix the iMac if possible and save up my pennies.