I will try to make this sound as simple as possible:

My Dad came down to visit for a few days, he is a GTA fan so we made a joint purchase on GTA IV second hand, got to the first loading screen, it froze. Re-booted it, got past the loading screen but htis time probably about 20 minutes into gameplay, it froze again. Kept trying, I might of tried another four times. But it wouldn't play without freezing. The next day, Took it back and decided to get a new copy of it. This time i got past the loading screen on the first gp, but maybe 30 minutes into gameplay i got a frozen screen. Yet again i kept trying several more times. Same result. I even moved my 360 to an even better ventelated spot, but still kept freezing:( So, the next morning i played it on my step dads Xbox 360 pro with a 20 gb hardrive (mines an arcade, with a 226 mb memory card) and it worked fine for over 2 hours. Sweet i thought, but I still can't play it on my own. So later on, iI decided to take back my GTA V game and get Gears of War 2 instead, Brand it home, 10 minutes in i got another error, but this time it was different, i got - This disk is unreadable - and it told me to either clean the disk or clean my lens. I knew it couldn't be the disk because it was spotless. Gave me lens a clean with a new lens cleaner disk my step dad has bought for the 360 and tried again afterwards, guess what? DISK IS UNREADABLE! OMG! So, my step dad suggested this was a memory problem, so i stuck his hardrive onto my console and got a good hour out of it. So now i know what the problem is, I need a hardrive. Later on a researched for a long time on the computer to see if anyone else has this problem, and if therew was a HDD - free solution. No solution. Next night after that, my step dad generously opened his heart and asked - I will give you my hard drive if you give me your Lips singing game with the two wirless mics, and your memory card. So i was like - SWEET!:twisted::) - trying to hide my smile. Ever since the hard drive instalation, it has been running fine, and i have no regrets on now having Gears 2 instead of GTA IV. I have now installed Gears 2 to my hardrive so it now runs quiter and smoother:)