Q: iMac display issues: black icons and folders, doubled up files in finder windows, programs won't update

HELP!

I'm hoping someone can help with my iMac 27" purchased in 2010.  From a previous discussion I started a year ago, I have had virtually nothing but trouble with it.  After the internal hard drive crashing, I was able to install OS X on an external drive and used it for a good 8 months to drive my iMac and I took it to an expert who has replaced it with a 3tb HDD, but at some point the display has gone crazy.  I am not certain if it is hardware, i.e. graphics card or video but I'll describe the symptoms and add some pics to see if anyone has seen this before and knows what the problem may be.

1.     90% of the screen was covered with white with just a portion of the desktop showing through.

        (This happened when I tried to reinstall OS X on the old HDD and the new one, but didn't happen when using OS X install disc)

2.     Files, folders, photos and some icons on the dock go black

3.     Files and photos double up when in Finder.  When I'm viewing photos, it shows them but all the previous panes and the photos are jumbled behind the

        photos.

4.     Anything I open in Preview is completely black, but ColourSync Utility opens everything fine.

5.     Programs won't update.  They will download onto my iMac but won't update.  Some say, because I don't have OS X 10.6.8 others just won't open.

        Including Google Chrome, opens with two tabs open but when I open a third tab the rainbow wheel just turns.  Skype won't open just gives a failure

        report.

6.     10.6.8 Update Combo downloads but when I restart the screen is covered in white - as in 1. and I have to reinstall OS X 10.6.2 to get the screen to               clear but then all the above problems continue.

7.     Sometimes videos don't display in Youtube, or the entire page other than the video player is covered in rainbow lines.

8.     Some buttons are blacked out on selection pages, i.e. system preferences, or saving screens

9.     Photos opened in FB go upside down and flicker all over the page as my mouse runs over them.

10.   Photos won't upload into internet editing programs.

I can't find anyone who has had the same issue in all of the discussions I've looked at, but it sounds like I have a video problem, but I wondered if it might be a software problem with flash or something.  Or I have something malicious on my computer.

I also have an iphone 5 that faulted with the iTunes and USB icons and won't fix after trying one of those R-Sims, and I wondered if somewhere along the line it may have even had a virus/trojan/malware that has infected my external drive and then my internal drive.  A reason I have thought that is because I have used my daughter's MacPro laptop and plugged in my external drive and I noticed the windows in hers were starting to do the same thing.

And on top of all that, my fans didn't work for over a year and I had to place an external fan at the back of the computer or it would overheat and shut down, but after resetting the SMU it started working again, but it is very noisy. I have tried to download SMC Fan Control and although it downloads to the HDD, it won't open.

Sorry its such a long post, I do hope with all of this information someone can help me.

Thanks so much.